Stefan Beller, the original author of that code, wrote an explanation at one point. But it seems to have gotten lost for some reason (probably due to the Github organization migration). Luckily, I saved the email, so here's what he wrote: https://gist.githubusercontent.com/dorkster/ddd19a0c91400ffd1c31c8ba18df...

I'll also attach the graphic he made, since I think it does an even better job of showing how the information is laid out. Granted, the description is a bit out of date since we don't have the 256x256 map size restriction anymore. Which is why the shift amounts are different in the current version.

The TL;DR explanation is that there are 4 critera for sorting. In descending order of importance:

• Which tile the object is on (red)
• Which row is the object on (green)
• The object's position within the tile (blue)
• Any additional priority information, such as when handling dead/alive or speical effects (yellow)