Mechanism Description
explains the arrangement and shape of an object in space. Such a description may involve movement, complex motions are better handled with the process description. Typically, the parts of mechanism description answer the following questions in order:
Process Description
explains the arrangement of a sequence in chronological order. In organization, it is similar to mechanism description, except that the "part-by-part" becomes step by step:
Process description includes sequence, instructions and procedure; however, only instruct if you expect your reader to perform the process you describe. Try to keep separate these two concepts: "How to do something" and "How something occurs" The first calls for instructions or procedure; the second, for sequence.
Classification
involves grouping things together (on the basis of similarities) and dividing them (according to differences). Classification assists in the complete consideration of a topic .Note the danger of Faulty Coordination.
Partition
is the act of dividing things into their component parts; very similar to classification, and an inevitable part of mechanism description and process description .Partition could be spatial (how each part looks) or functional (how each part works).
Definition
uses words to fix the meaning of a thing -- to make it "definite". The short definition (a paragraph or a single sentence) is essential to technical writing. For instance, the Mechanism Description and the Process Description each begin with a call for a definition. A definition answers the question "what is it?" Good definitions employ the following formula:
Sometimes definitions might be much longer than one sentence, in which case you are still trying to answer "what is it?" but will be using most other patterns to help answer it: you can define by describing, classifying, comparing, etc.