This piece of ASM makes tiles appear in the overworld border, to show if levels have one, two, or no exits, and also if said exits/events were triggered. Useful for tracking normal and secret exits in general.
It's recommended to use this in gamemodes D and E. Also it's currently incompatible with the 'more OW levels and events' patch.
A rather useful resource, especially for larger hacks. My only complaint about it is that it could be made a bit more user-friendly, especially with the positioning of the tile (a direct VRAM address isn't that easy to figure out) and the calling of the routine (you need at least some ASM knowledge to JSL to it properly).
The routine tells you to place it in main code. I recommend you place it in NMI code, especially if you're not using it for something else (or animating any tiles). The write to VRAM directly uses the PPU registers and works in main code by dumb luck (the NMI routine in Super Mario World almost always exits quite a bit before the end of V-Blank, so code run at the very start of the frame will still be in V-Blank).