Concert seating chart, need help!?!

I'm might go to a good charlotte/boys like girls concert in Philadelphia!. I'm probably going to get the tickets for section GA!. On all the seating charts I've seen they only have reserved areas and general admission areas!. When buying the tickets section GA and general admission are two different things, so I don't get why GA isn't on the seating chart!. Please tell me if there is an explanation for this or if there is a difference between GA and general admission seating!. Thanx!!!Www@Enter-QA@Com

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