You must register for classes every quarter that you are at UW.
Grad students have a particular number of credits that they may register for. Elise will know—it differs for first-years, by fellowship, for international students, and so on.
Usually, students register for one course per quarter (if they are still taking classes), the CSE colloquium, a PLSE reading group, and the PLSE group meeting.
You can register for a seminar or reading group for 1, 2, or 3 credits. The main reason for the flexibility is so that your schedule for the quarter has the correct number of credits.
You can also register for Independent Study with a faculty to fill out the correct number of credits. You can find the code for your advisor on MyCSE.