Hidden Pocket armor allows you to hide weaponry (or potentially other items) on yourself in case you face a search by someone. If you remember my Hidden Weapons (http://kamalpoe.blogspot.com/2011/03/crimmor-hidden-weaponry.html), Hidden Weapon class weapons are accounted for (they hide themselves, and so do not count against the hiding capacity of the armors).
Picture one: Wearing Hidden Pocket leather armor, the guard doesn't see the short sword in my inventory. You can see in the message window the hiding capacity and capacity used of the hidden pocket items I am wearing. Hidden Pocket armor by itself can hide one small, and two tiny, weapons.