Bag World is the plane in our multiverse where all Bags of Holding (which includes Handy Haversacks) are connected to. Whenever someone puts an item in a Bag of Holding, it goes there. Whenever someone enters a Bag of Holding they go there. For this reason, in the Harvest Moon Campaign, if someone enters a Bag of Holding, they are not suffocated. They have plenty of air and, if there's food and water in the bag, they can subsist on that. Unfortunately, unless the bag is emptied or the person is pulled out, they have no way of getting "out of the bag" as it were.
The world is a massive place which curves in on itself. It is widely thought that Bag World is actually a super massive Dyson Sphere, though no expeditions to Bag World have resulted in 100% proof of this since tunneling through to the other side presents logistical issues and also serious dangers. There is some circumstancial evidence of this in that the sun never sets and there are three planets which lazily cross the sky above Bag World along the same orbital plane.
For the most part, Bags of Holding safely contain their contents. Items within the bags are held in Bag Zones on the surface of Bag World. Bag Zones are difficult to find since the magic used to create the bags themselves causes the zone to be "nearer" to our world just enough to resist magical detection. Another benefit is that our world is a micro-fraction the size of bag world. Bag Zones are so far apart that accidentally stumbling across one is a rare event. Additionally, most of the terrain in Bag World is desert. Any denizens of Bag World face extreme difficulty in plundering Bag Zones deep within deserts millions of miles across.
Additionally, most (95%) bag zones are also pocket realms which, although visible from the outside, are impenetrable by physical means or teleportation of any kind. The only way in is through the bag aperture. Some Bag World pirates insist they've found a way to crack through this protection, but these accounts are not taken very seriously by scholars.
In all, the odds of losing your items within Bag World are so remote that the convenience of the bags far outweighs any of the risks. Some Bags of Holding (1 in 10,000) do have the interesting drawback of their Bag Zone being underwater. Items placed in these bags are wet when retrieved.
There are some garden spots of Bag World, mostly where huge bodies of water still simmer in the hot sun. These territories are populated and have their own societies, some strange and some familiar. Bag World denizens rarely bother trying to enter our plane due to the difficulty involved. There are some Bag Zones anchored in these areas. Some might say that these bags are considered useless for purchase, but they are sought-after by those who would profit from Bag World commerce and travel. These bags can go for ten times as much on the open market as a normal Bag of Holding, though testing their authenticity can be a challenge.
It is thought that Bag World's plane of existence has been connected to ours for quite some time since the races of its denizens are all the same that can be found on Earth and the primary language spoken is Common. It is surmised that this is due to the Earth being our Multiverse's Lynch-Pin (please see the Cosmology article for more information on Multiverses and Lynch-Pins). Others believe that the structure of our Multiverse is a bit different than we currently believe it to be. They hypothesize that the multiverse isn't an infinite stack of layers, but a loop, helix, or even ribbon which allows for not only adjacent planes, but opposite planes. Philosophical opinion leans towards the former theory as there is little proof of finite planes.
Creating a Bag of Holding
When creating a new Bag of Holding, roll a d100. On an 1-85, your bag is protected within a pocket realm. After that, roll a d1000. If a 1 is rolled, your Bag Zone is underwater. If a 2 or a 3 is rolled, it is in a conspicuous place inside of or within 100 miles of a habitable non-desert area. Bags in a conspicuous area without the protection of a pocket realm are worth 1/10 their value on the open market (though they do have a habit of being sold to a long line of unwary adventurers until someone with a conscience simply destroys the bag). If a 1000 is rolled, roll again. If you roll another 1000 on the 2nd roll, your Bag Zone is directly inside a major town or city in Bag World. Bags not within a pocket realm which are close to or inside a major city or town are worth at least ten times as much as a regular Bag of Holding of its type.
Interaction Between Bags of Holding and Other Extradimensional Spaces
In Harvest Moon, Bags of Holding use the standard rules for becoming inert when placed within other spaces. The connection between a bag and its zone is only made through the firmament between our world and Bag World. For this reason, this rule is expanded to include any other plane. For example, a Handy Haversack would not work at all in Arcadia or the Ethereal Plane. It defenitely doesn't work in Bag World itself.
Placing a Bag of Holding inside a Portable Hole does not destroy either item. Bringing a Portable Hole to Bag World has no negative effect because the physics behind the Portable Hole are the same in Bag World as in our world. (Portable Holes are actual portable pocket realms unlike the bags which are simply portals to Bag World.)
The only real oddity that happens is when a person, somehow, is shifted into a Bag Zone while holding that zone's bag. This renders the bag itself inert, but the zone remains. According to any current knowledge, if the Zone is a pocket realm, that person is trapped there forever.
Destroying a Bag of Holding
If a Bag of Holding is pierced, the bag is destroyed but the items remain in the location of the Bag Zone. Unfortunately, the zone itself is destroyed leaving the items inside exposed to being found and pilfered by Bag World residents.
The full information in this article should be considered unknown to your character unless they learn it through gameplay or have 9 ranks in Knowledge (Planes) and Astronomy (or 6 ranks plus skill focus). You may have partial knowledge of it or investigate the topic at your DM's discretion.