These are general purpose butt packs from Eagle Industries. These large butt packs are a great way to carry extra gear or supplies while out and about.
The large main compartment has a water resistant compression sleeve, similar to issued large rucks. There are three exterior pockets on the front with buckle closures and a top zipper compartment.
The butt pack is MOLLE compatible and can be configured to be worn on a battle belt, put on the outside of a ruck for extra storage or worn on the back of your kit as a backpack.