Powerstack batteries which are special because they use the pouch cells, much like those in mobile devices like phones, laptops, drones and more. These are a huge upgrade over the ordinary XR battery packs which contain either 18650 cells or 21700 cells, both of which are cylindrical.
Even when packed into the most efficient arrangement (hexagon positioning), there is still much wasted space between each of the cells. The DCBP518 5.0Ah PowerStack battery (and previous compact version) solves this by using flat pouch cells, essentially cuboid shapes.
These have no wasted space between cells, and minimal wasted space between them and the battery housing.