Bumfuzzled about _levels

I was handed a project some time ago, that someone else had started. It’s all done using AS1, and some of the various snippets of code include stuff like this:

_level0.mod_num = 1;
_level0.start_page = 1;
_level0.end_page = 6;
_level0.button1._visible = 0;
_level100.footer.pauseplay._visible = 0;

My question is, how to I figure out what layer these commands target? The person working on the project originally didn’t have the decency to give the layers any names that made sense, and I’m kinda lost. Many thanks to anyone with an answer.