The orlop deck is the lowest above the hold. In a three-deck man-o-war it was below the lower gun deck with its 32-pounders, so not terribly restful in battle for the unfortunate victims thereof who were patched/sawn up down there.

Also the site of the (tiny) cabins of the Warrant Officers: carpenter, sailmaker, gunnner etc.; the magazine and (midships & fwd) the cable tier (as mentioned above).   Pretty similar in merchantmen I imagine, but without quite so much noise, blood and gunpowder. Or so many cabins, since they typically had only a Master, Mate and Second Mate.   So I guess it was used for further stowage space. Maybe there was no deck there - just a massive hold from the ship's bottom up to the upper deck. Probably depended on what the ship was designed to carry.

You can now tour the orlop and hold on the VICTORY at Portsmouth, a tour well worth doing.

Steve