Free form, and the way it was played in the other site was that there were original characters, if you so choose, but there are other options as well, like in the recent movies. For example, I have to characters that I played there. One was the daughter of Psylocke and ArchAngel, the other was something akin to Daredevil, only younger, and female.
It's pretty open as to what the timeline is, it just needs to be later in life of the original X-men. For example, for my first character to work, it would have to be in a time where Psylocke and ArchAngel are old enough to have a 22 year old daughter. If Psylocke is even considered alive, at this point. *Chuckles*