Internal organization architecture and job description of the applying organization.
They are not bound by geography, job description, institution, or demographic.
Look at the birds, free and unfettered, not tied down to a job description, careless in the care of God.
Then you can map each job description to one of the user categories, giving you a general understanding of user distribution and anticipated workloads.
And, if you want steal a great job description for an intern and recruitment method, check this post out from the ACLU of Northern California.
Condoleezza Rice was a dyed-in-the-wool realist who had famously observed that the 82nd Airborne's job description did not include escorting kids to school.
The paper wrote that a job description for the post of Chikurubi prison hangman included, "dexterity, ability to tie a knot, hard heart and anyone prone to mercy or hesitation need not apply."
