Actually, the +31 -1 stuff is used almost everywhere.
It's because the game adds padding regions to the sides of the map (If I remember right, they're used for blight expansion, letting units exit off the edges, etc) and the mission designer could also use them to hide things that will be used later, like units, possibly even whole bases =)
So in effect (0,0) is probably still a real place on the map, but not visible to the user.
(31,-1) is the upper left corner of the player-visible map.
We haven't documented the padding regions at all, they seem to be different for different map sizes if I remember right. The best way to find out whether they're usable is to just test. If it doesn't work the game will crash trying to do something in that part of the map.
World maps, I'm not sure if they have left-right padding. I don't think they do. They might have top-bottom padding though.
Just play around with it if you're really interested in putting stuff outside the map.