The Campus Fee Grant is a need-based award. For many students it truly is the proverbial free lunch, but there are exceptions. Two main factors lead to a student's not having enough financial eligibility -- or worse, seeming eligible at the beginning of fall term but losing eligibility later and having to return the money. One potential problem is if much or all of a student's support comes from fellowships instead of TA or GSR positions. The other is having an unusually large Expected Family Contribution to the financial package, around $18,000 or more. Usually this happens because of significant savings or a spouse's income. The two factors can also interact, e.g. there could be trouble if a student has a medium-sized fellowship and a medium-sized Expected Family Contribution.
To be sure that you won't run into trouble, go to MyAwards. You should see an "Estimated Cost of Attendance." Ignore the tuition and health insurance items, which will be nearly or entirely paid through your TA or GSR appointments or by the department, and will count as part of your financial aid package. The remaining items (books, room and board, transportation) come to over $18,000. As long as the sum of your Campus Fee Grant, Expected Family Contribution, and any additional loans or fellowships, departmental or otherwise, stays below this number, you're fine. If you're one of the rare students where it's even close, you can go over the numbers more carefully with the Grad Coordinator. The key is that the "Remaining Cost" line needs to stay positive (or where it would be positive if you declined the $20,500 unsubsidized loan) even after factoring in any fellowship stipends and your TA/GSR fee remissions and health insurance for all three quarters. If you turn out not to be eligible for the $600, then depending on your exact situation it may be possible to get your numbers adjusted so you are. Talk to the Grad Coordinator as soon as you notice the difficulty.