The short version is this. Saber, Archer, Lancer, Berserker, Caster, Assassin, Rider--any heroic spirit throughout history who has ever committed their names to records of note qualifies for one or more of these classes. The Knights of the Round Table, Gilgamesh, people like that.

[...A couple of which he knew a little too well.]

Seven mages call forth heroes from the past in a seven-way battle to the death in hopes of attaining a miracle.