caulk is like a texture for a side of the brush that the player can't see, it makes it so it doesn't take up as much kb and lessens the lag.Like if the player won't be able to see a certain side of a brush you would put caulk there so that it doesn't use extra kb for that useless texture that the player can't see.
Hint I believe lessens lag as well so that the player cannot see all that isn't in a hallway or room.I'm not to sure if that's right though, just a guess.
Origin is used for doors, its kinda like the hinges of a door.