Like what was already stated.
Originally, the Servant system was designed to summon only the souls of "true" heroes - benevolent, honorable, and selfless protectors of humanity and justice. The corruption of the Holy Grail in the third Holy Grail War is the reason evil spirits can be summoned. These entities can still be considered to be Heroic Spirits if they were widely worshiped, respected, or feared in life.
Also like what was said before, again; It's a different system being used, so some terminology changes would be fine by me as long as the general meaning is still the same.