Traps are solid wall containers within the tank. Liquid is held within the trap by a porous element covered trap inlet window. The porous element offers propellant retention at higher accelerations than a typical sponge. The PMD shown to the right is a cylindrical trap with a porous element wrapping around the cylinder (shown in green). A sponge is also illustrated outboard of the trap.
Traps are not passively refillable during zero g coast like sponges. This makes them unsuitable for repeated orbital maneuvers like stationkeeping where sponges are often used. Traps are typically used for once in a lifetime maneuvers such as contingency despin, station change, or launch gas retention. Traps can also be used to compartment the tank. This might be useful to minimize PMD mass and complexity or for CG control at high accelerations.
While most traps are not refillable, a trap can be designed to refill during a high acceleration settling maneuver. These refillable or venting traps are also called start baskets since they are most often used to provide propellant for the ignition transient associated with large axial burns. The first PMD for the Agena upper stage was a start basket.