I am also really curious. I get a new tab for a new agc file, but it doesn't show up in the sidebar. Also, it seems like the sidebar only has one agc file under each project, which doesn't really make any sense.
I'm trying to have separate agc files, one for the main game, another to load all the maps, and another to load the default object properties, etc. That way in the future, if players want to mod, they can easily find the object properties agc, separated then into different functions and they won't have to mess with the main code.