A bag of holding is an enchanted item keyed to its owner that is found both on Praxis and Ithus. The cloth or hide baglet attached to its aperture is more for decoration than anything else as the opening is the important bit. If one reaches inside of the opening (and does not trigger a condition that prevents access) the bag reveals itself as an extradimensional space many times the size of what one would expect. Items dropped in are deposited gently on a shelf or pile (depending on design) and can be retrieved at a moment's notice.
Creating a bag of holding is not as easy as it seems, therefore these are usually not as common as an ARE. Furthermore, the skill of the crafter greatly influences the available space inside as well as additional functionalities. For example, a C'try Mouse field worker who has studied magic enough to progress beyond cantrips might create one with enough space inside to store a rain barrel. Whereas the one owned by Lady Eiraiha is said to have enough space inside to easily fit a city in. Plus, she can store items in several piles by willing them there.
Either way, once someone has created or obtained a bag of holding they have to attune themselves to make the most of it. Experienced spellcasters can expand a bag they created while younger, but most prefer to start anew and sell the old one to one they judge to be worthy. The magic rituals tought to make them are strictly adhered to as failed experimentations have caused severe incidents, the least of which was one Mamonme losing the arm she'd stuck inside to a hungry bag.
Furthermore, all but the oldest bags have gained a mandatory seal that prevents them from being opened during teleportation or while perusing portals and gate spells. This was implemented after Granny's Sabbath on Ithus shared knowledge on the effects of such unwise actions.