Defining your audience helps you answer the "Who are you writing the document for?" question and part of the "What level of detail are you going to use?" question.
A good audience definition should describe the type of person that makes up 80-90% of the user population. Most people will easily understand your document, but some need extra attention. These people usually only make up a small percentage of your audience. If you wrote a document to cater for all levels, 20% of your effort would cater to 80% of the audience, and the remaining 80% of your effort would cater to only 20% of your readers.
Your audience definition can contain the following information: