What they already know affects what you can leave out. What they need to know determines what you include. Wha t they want to know suggests the order and emphasis of your writing. To be most effective, you may need to produce three different reports for the three different audiences. Structure Writing is very powerful - and for this reason, it can be exploited in engineering.
Structure is used to present the information so that it is more accessible to the reader.
In all comes down to the problem of the short attention span. You have to provide the information in small manageable chunks, and to use the structure of the document to maintain the context. As engineers, this is easy since we are used to performing hierarchical decomposition of designs - and the same procedure can be applied to writing a document. A paragraph should convey a single idea. Whole reports can be written with them almost exclusively and you should always consider using one in preference to a paragraph.