What is the difference between sessions A and B in regard to a semester of college at a university.?

I was filling out a form for financial aid for the university that I am planning to go to (LSU) and I was asked the question " Will you be attending school for the summer session's A, B, or both. At the college I've been at, there were no option like that and the school's catalog dosen't answer my question either. What is the difference between session A and B for a semester of college? Hope someone knows what I'm talking about.