Rabbits are territorial, and divide the world into personal places only for them and their closest associates, public places, and places owned by others. Some rabbits are more territorial than others, but many rabbits will be very angry if you reach into their homes (always a personal place), especially while they’re inside. If you do so, anticipate getting charged at, pounded hard with pointy-tipped paws, and even bitten. If you’re going to mess around in a rabbit’s home, it’s usually best to do so while it’s visiting elsewhere.
Rabbits recognize when a space is owned by others, and where they’re not allowed to go. Of course, places owned by others just beg to be visited whenever possible, although it’s understood that one might get chased back out. After all, perhaps today one might be allowed in. Testing is the only way to be sure. Accordingly, you may set aside rooms where rabbits are not allowed, but don’t expect them not to try to go in anyway. They might actually hesitate at the door, though, to make sure you aren’t inside and watching.
The main way that rabbits indicate they own a place is by placing some big dry droppings (pellets) there. Those are in addition to the usual ones placed in the litter box! A rabbit will always place some pellets in its home, of course. If you have several pets and your rabbit feels somewhat threatened by them, it may also stake out some other places around the house or apartment, particularly in corners. Unfortunately, there’s no way to really prevent this except to make your rabbit feel very secure and in charge, so it doesn’t feel it needs to aggressively mark its space. Wiping the area with some vinegar may help.
You may notice a rabbit brushing or rubbing its chin against things, particular on the edges of objects, corners that stick out, or any unusual object in its path. Rabbits have scent glands under their chins that they use to mark their territory and objects they own. We can’t smell the scent, and it doesn’t stain or cause visible marks. There’s no reason not to let them mark everything in sight, including you, which many rabbits will indeed do. Trying to do the same by rubbing your armpits against objects to mark them won’t impress the rabbit or anyone else.
Unneutered/unspayed rabbits will also spray urine to mark territory, as well as “objects” they own (sometimes including other rabbits). All pet rabbits should be neutered or spayed, which will generally stop this behavior. Wiping an area with vinegar will help kill the urine smell and may cut down on spraying there.