The term “helpless” is used in multiple places throughout the books, including in the description of stinking cloud. However, “keyword mechanics” were not really a thing in AD&D, and so some interpretation is needed to establish a defined meaning for the word. Since helplessness is very deadly, it is critical that everyone at the table has a solid definition.

Helplessness is not quite as bad as being held (which includes magical sleep), because a helpless creature can often still move.

An attacker targeting a helpless opponent during an active combat automatically hits.¹⁾

However, if the attacker is classed, they my choose to make an assassination attempt, regardless of their class. In such a case, regular weapon damage is still applied on failure. So unless an attacker does not want the target dead, this is always the better option, since it automatically hits as well.²⁾

Depends on what caused the helplessness. In the case of a grapple, of course the target can't use their MV. But in the case of stinking cloud, nothing is restricting movement.