If you are receiving a tuition waiver (typically it is applicable for most Ph.D. students and sometimes supported M.S. students) , you must be aware of the following rules:

1. If you are not registered as a full time student then the tuition waiver (and, in fact, any other kind of support) is canceled.
2. Tuition waiver is paid only up to the minimum amount of credits that you are required to maintain in order to be in full status. That is, if you are getting full tuition waiver, it goes up to 12 credits for G1/G3 students, up to only 9 credits for G2/G4/G5 students.
3. Paying for any extra credits is the responsibility of the student. Note that every semester there is certain deadline after which the University charges tuition even if you drop the extra credits. Please be aware of the appropriate deadlines.
4. Registering but not attempting a course (receiving the NR grade) is treated the same way as if the course was never registered for.