These are what I was wondering- if it was based on some sort of value, or the state of the attacking player. Seems to be somewhat both (seems like you can't tech in situations where you can't Burst like during Overdive/super moves), but in general it seems to be a value- like hitstun or blockstun or stun damage, certain moves cause X amount of frames where you can't tech (or hitstun# +X, etc).
In Street Fighter Alpha 3 for example, it's based entirely on states- when a move is active you can't tech, but during recovery time you can, something like vs overdrives, but all the time