Global Entity Names
A global entity name allows objects to "exist" in more than one level.
"An easy example is you can see a building, but you can't get in. So you just shoot a bunch of holes, throw grenades, break the glass, etc. Now, if the inside of the building was a different level, and you used global names on all the things that you could change (glass, decals - stuff like that) then it would all be in THAT level, even though you weren't in it when you made the changes!"
-John Guthrie (Choryoth), level designer