At the risk of channeling Captain Obvious, the game manual suggests the following; and while the PBP does its best to keep secret who it is the "fifth defender" is guarding (why that is I have no idea) this is consistent with my experience:
Inside Box-and-One: The fifth defender focuses on the opponent's best inside scorer. Normal pace.
Outside Box-and-One: The fifth defender focuses on the opponent's best outside scorer. Normal pace.