Being held is worse than being helpless, because the target is basically helpless AND unable to move. Any creature that is held is also helpless. “Held” includes magical sleep, such as from the sleep spell. It does not include regular sleep, which is only “helpless”.

If combat is in progress, the attacker gets “double max-damage auto-hit attacks”: attack routines used against the target are doubled, and every hit scores maximum damage.¹⁾ In this case, because any creature that is held is also helpless, any character regardless of class is able to make an assassination check against the target (for one or both of the attacks, at the attacker's option), with failures causing max damage.

However, if the attack is being made outside of combat (i.e. when any living enemies have been disabled, fled, or otherwise pose zero threat to the party), the attacking character may instantly slay or bind a helpless target at a rate of one target per round.²⁾³⁾