[quote=McFazzer]
 It says "tbd by GM", does that mean we only know what it is after submitting a character?
[/quote]
We talked it over when we were making the sheets, and we didn't want everyone having a phoenix feather core, so to add variety we will be choosing cores, yes.

Edit: It is very possible to end up with either of the three types of cores. I thought I would clear that up.