Drag anything ontop of clothing which contains an inventory.

When the clothing is inside of an inventory you can drag any item on to it and store the item inside of the clothing inventory.

Items stored this way can still be used via the hot bar.
This works for items already in an inventory IE:. A bandage inside a med bag can be places into a clothing article that is inside of your backpack, and you can still use the bandage via the hot bar.

Once an article of clothing has items inside of it's inventory it can not be moved in the backpack but can still be placed onto the ground to retrieve the contents inside.

This essentially makes any clothing article with 6 slots equivalent to a Small Protector Case.