As Matt has said, essentially what happens when you get out of the coursework portion of your PhD is that you're now working on research. In a lab, at your dini

As Matt has said, essentially what happens when you get out of the coursework portion of your PhD is that you're now working on research. In a lab, at your dining room table, etc. But it's important for a number of reasons that you remain a student. It allows access to student services, you maintain full enrollment so student loan related repayment things don't trigger yet, you're eligible for student health insurance, etc. So you need to take whatever a "full load" is, and tuition is billed accordingly. Except the university knows this is silly. You're not taking classes with instructor time, or in need of a classroom, TAs, etc. So you get a tuition waiver. Basically, the sum total of this is to allow for an accounting workaround because there's limited ways to express "You're a full time student taking no classes".