A fullblood is 100% of that breed - of the old country bloodlines.
Purebred is 93.75% of the "old country" bloodlines but has another breed in there, even if it is very very little.
Simm's I remember had a big sqaubble about this a couple of years ago. Someone wanted to call their's fullblood but yet they were black. So even though they are very high percentage Simm they are called purebreds. Same with Chianina, Salers, and just about every breed.
Now in some registries the hybrid can be purebred after 3 generations (on both sides). Such as Chiangus and Simangus.