A dimension that can be used to store energy, items, weapons and any thing else one desires. Even whole
army's. The average size is usually limited and not usually as vast as a proper dimension which houses a solar system sized reality with in a main universe. Though this rule may not always apply. Some pocket dimensions can be the size of a proper universe depending on how powerful the being is who made it or where it drew it's power from in making it.
Also has other uses beside storing something. Can be used to connect from one space to another like a sort of bridge for Dimensional Travel . Can be used as a Sealed Dimension in order to trap desired targets in. Another use could be for creation of new lifeforms. Could be used to replicate another reality or area of another reality. Or to build and house something like a maze inside like King Minos's Labyrinth .
D's Left Hand has a stomach that is probablly a pocket dimension, although it might have expanded to a proper dimension, or well past universal size. It has shown no real sign of a limit of how much it can house. The Elder God created a space that absorbed the entire universe. D was thrown into the Milky Way, and Left Hand was able to absorb and negate it.
The Sacred Ancestor's Army may have divided a portion of it's army to having been held inside one in wait for if the Living Fortress ever went back into operation. It may have been too much for that, but they may have a higher level form of Miniaturization or the means to create a proper sized dimensional size to be housed in.