Cost of Attendance (or COA) is a calculation that every school with federal student aid must do. The simplest answer as to what this is can be explained as being an estimation of how much a student's attendance for the award year will cost them. Some things that are included are: tuition and fees for the year, transportation, personal expenses, room/board, etc. Some special exceptions can be made by some schools; however, our COA is adequate so we don't make exceptions. If you use the COA as only a tool to figure out how much it costs to attend school, then you must keep in mind that this is only an estimate, and really only your own personal budget is the real answer.
The other use of the COA is for your total Financial Aid: all of your Financial Aid cannot go over your COA. If you add up anything can be used to pay for your tuition (grants, loans, scholarships, self-payments, state funding, etc.), then the maximum you can receive, at the most, will even out at your COA. The reason for this is simple: if your COA is how much it costs to attend school, and you get an amount to go over that, then you aren't using your FA to pay for school. If you aren't using the money for school/education-related expenses, then you are not entitled to it.
The purpose of this is to keep people from borrowing much more than needed for school. Stafford loans have loan limits on them, so even if your COA is 20,000, your first year maximum (Subsidized and Unsubsidized combined) would be 9500 for Independent students and 5500 for Dependent students. But a PLUS loans's limit is the lesser of how much the parent chooses to take out or the COA. If a student receives no other FA than a PLUS loan, and the COA is 20,000, then the parent can take out a 20,000 PLUS loan. But if a student receives 5,000 in Pell Grant, and the COA is 20,000, then the maximum PLUS loan can only be 15,000. If a student's Financial Aid goes over the COA, then a student is considered to be overawarded, and a refund has to be made by the school. If excess funds have already gone to a student when the school finds out that they have been overawarded, then the student will owe that money back.
So Cost of Attendance is a little more involved than just simply how much it costs to go to school. Usually COA issues don't happen very often; they usually occur if something has changed with the student (dropping classes, testing out of classes, etc.). If anything changes from how your enrollment was originally planned to be (you were supposed to be full-time for 4 quarters, but instead you are half-time for one of them, etc.), you really need to stop by the financial aid office and make sure they are aware of the change.