Each numbered folder usually holds a self-contained module or a snapshot of the code at that point.

If you find a file claiming to be BFB_17_source.fla, check these details to avoid fakes:

The "Screamer" era of YouTube is over. Taking the raw .FLA, rendering it, and uploading "BFB 29 (FANMADE)" is plagiarism. The source files are for derivative works.