According to Hayes and Flower , there are three main areas of the writer's domain that relate to the writing process. The task environment is everything beyond the writer that influences the writing task. The writing assignment--topic, audience, the writer's motivation, and the existence of portions of text are examples of task environment. The second element is the writer's long-term memory. Issues such as the writer's knowledge (about a variety of topics) and writing plans are stored in long term memory. The third item is the writing process.
Hayes and Flower  define three general phases of writing operation. These phases are: planning, translating, and reviewing. Planning includes the sub- operations of generating, organizing, and goal setting. The subprocess of reviewing are reading and editing. Planning involves retrieving the relevant information from long term memory and the task environment. This information is used to set establish goals and to develop the text that will satisfy the goals. Translating is taking material from long-term memory in accordance with the writer's plans and goals, and formulating sentences with it. Lastly, in the reviewing operation, the goal is to improve the quality of the text produced during the translation process.
The function of the generating sub-operation is to retrieve information from long-term memory that is most relevant to the writing task. The memory is first probed with information about the topic and audience. Each retrieved item becomes a new memory probe. Associated items are retrieved from memory together. This process is broken once a non-relevant item is retrieved. This break in the process requires the writer to 'restart' the search with a new memory probe. Once an item is retrieved the writer may choose to produce a note about the particular item. This note can be a word, a phrase, or a complete sentence. The second sub-operation of planning is organizing. Organizing entails selecting the most useful material retrieved by the generating process and organizing them into writing plans. These may include what topics the author wants to address as well as the sequence of topics. The final sub-operation of planning is goal setting. In this sub-operation, some material from the generating process are criteria that are used to judge the text in terms of meeting the goal specified. Some of the criteria are used later in the editing process.
Reading and editing are the sub-operations of reviewing. In these sub- processes, the writer examines any material written with the goal of correcting anything that would prevent the text from meeting the established goals. This includes correcting grammatical errors, altering the contents, and improving the clarity of the writing.
These processes indicate that the writing process is controlled by goals. The differences in the way that individuals establish goals accounts for the differences in writing styles. For example, some prefer to produce a draft document and then review and revise. Others prefer to write and edit at the same time. Further, the generate and edit modes can occur throughout the writing process, that is the writer will make use of ideas that arise during text preparation.
As mentioned earlier, there are constraints that can restrict the writing process. Some of these constraints are set by the writer, others are imposed by existing subsections of text. Constraints can be useful in that they can cause the writer to restrict his/her search of memory when generating text, they can also improve one's focus on the task. However, constraints can impose cognitive demands. Ideally, the writer should attend to all constraints. This, however, does not occur because of the limitations of short-term memory or conscious attention.
According to Flowers and Hayes , there are three major areas of constraints. Each ares includes (or builds upon) the constraints of the previous one. The first is knowledge, which can also be a resource. Knowledge becomes a constraint when it is not suited to the specific writing task. Unorganized thoughts and perceptions have to be transformed into related knowledge. The second constraint is expressing knowledge in text form according to the grammatical and syntactical rules of writing. This area is referred to as written speech. It address the difficulties in acceptably expressing thoughts in writing, given the imprecision of spoken language. Lastly, the writer is constrained by the purpose of the text and the intended audience.
There are solutions for reducing the burden of managing multiple constraints. One can selectively ignore some constraints by recording an idea without including it in the text. Another option is to partition a problem into sub-problems, and set priorities based on the sub-problems. One other option is to use routine or "boilerplate" procedures (which can reduce some of the cognitive overhead involved in remembering structure and format details for example). In general, planning is the best solution for managing multiple constraints.
There are several types of plans. One type is a plan for responding to goals of the writing project. A second is deciding what will be included in the text or preparing an outline. Another is composing plan that includes strategies to assist the writer though the writing task, such as "brainstorm".
Another model of writing strategies is provided by McGowan. The first stage in this model is pre-writing which is generating ideas and developing themes to express the ideas. The next stage is writing which involves physically creating the text. Lastly, post writing which is comprised of evaluating, editing, and revising the text.
Sharples and Pemberton  present an enhancement of the cognitive model of writing. They believe enhancements are needed because the model is not specific on how text is represented at each stage of construction, there is no distinction between mental processes and those same processes performed on an external medium, it does not account for the ways writing is affected by different media, and there is no framework for the various approaches or strategies. The focus of this extension of the writing model is the ways a writer uses text to facilitate the production of the document.
The enhancement pertains to text and the notion that it differs in the functions and in the operations that can be performed on it. According to Sharples and Pemberton there are three types of text item. One type is instantiated items. These are a series of words or "fleshed out" text that is considered to be connected prose. Another type is uninstantiated text or a place-holder for text to be created. This item is referred to as a idea-label. It can be broad ('this needs more details...') or specific ('add information about...'). If the idea-label relates to a familiar topic, the writer has little difficulty instantiating it. However, labels for broad categories require more planning and fleshing out. The only difference between instantiated text and uninstantiated text is the function. The third type is annotation which is a comment or other item used to express the writer's intentions or assessment. This text item guides the revision process.
Another area of concern is how the writer organizes these text items on paper or a screen. There are three general types of views. Items can be unordered such as comments generated during brainstorming. They can be non-linear such as mind- maps or idea networks. Non-linear items are the intermediate stage between ideas in memory and finished text. Linear arrangement refers to a series of words.
The task of the writer is to produce linear instantiated text. This process is cyclical. Instantiation and views characterize the author's representation of items on an external medium. This enhanced model allows a distinction to be made between processes performed on an external medium (such as writing notes on index cards), and those performed mentally (for example, recalling information from memory) or those bypassed entirely. This extension of the writing model also includes a decomposition of writing techniques into a sequence of core operations. This requires an extensive discussion and will not be addressed in this paper.
Sharples and Pemberton  define the sources of constraints as: task related such as the purpose of the document, the audience, the required length; external resources such as available books and articles; linguistic knowledge and vocabulary of the writer; and the material already created. Constraints can also be categorized by function and characteristics. Content constraints require direct probes of long-term memory, and ensuring that the final product fulfills goals. Resource constraints relate to the external resources for example, key phases to use or words to avoid. Boundary constraints relate to the length of the document, the maximum number of words and the like. Structural constraints provide the framework of the text, and need to be appropriately general. Lastly, contextual constraints ensure consistency for example, the same abbreviations are used throughout the document.
The properties of the media used to facilitate the writing process influence the strategies of writing. Writing notes on index cards, allows the writer to re-organize notes in ways that cannot be done with a single sheet of paper. In the case where a document is produced on a word processor and notes are prepared using that word processor, they can be easily re-used in the final document. It is important to note the properties of the media that support a giving writing process. (Some characteristics are the medium is: browseable, portable, re-orderable, manipulateable, usable in the final product, permanent--available for re-use, supportive of annotation, indexable.)
These writing strategy models indicate that writing is goal directed and controlled by multiple constraints. The writer defines and manages tasks by setting constraints. Writing constraints include the topic, the audience, the purpose, document length requirements or restrictions, available resources, such as memory and ideas, and draft versions of the document. The process of constructing, storing, and applying constraints places severe demands on memory and processing ability. In the case of routine writing, a solution is to use pre-stored text types of valid documents, such as a successful research report. Planning is one effective means of managing constraints, while fulfilling the goal or purpose of the text. Further, writing is iterative, not a step-by-step process. The writer may proceed through a series of sub-tasks several times before producing text that is acceptable. Time is spent generating ideas, however, the act of writing can also provide insights. There is no single best approach to writing.