[General Question]

My admission letter for the MS in Computer and Information Science program Upenn states that after February 15, the order of admission remains valid, but my place in the class is not guaranteed. Could you clarify what this means? If I accept the offer later post February 15, will my spot still be secure? As I am waiting for other schools as well. And if pay acceptance fee and accept the offer for now , later can I say no to them? Or else it will create some problem for me.