The answer to this question represents a vision of what a student should know and be able to do. Creating that vision is one thing; helping students internalize that vision is quite another.. Of all the questions listed above, establishing a vision is perhaps the most important.
List all the possible audiences and then figure out which audiences are most important.
Is the purpose is to help students understand themselves? Or, is it to serve a evaluative function?
There need to be some defined readers of the portfolio that will be receptive to examining student work.
What information will be collected in the portfolio?
Will you collect some information, in addition to student work, that helped to put student work in context. Most of the teachers ask for some kind of self-reflection, where the student explains why a particular piece fits in the portfolio.
The student work itself can have multiple components. Do you want to encourage the collection of multimedia pieces? Do you want to see works in process (rough drafts, early attempts at experiments) as well as completed works?