This is a neat tidbit of information I found that's actually incredibly useless except to anybody that wants to poke around inside of BYOND a bit:

When manually editing a DMM file, you can modify a type like so:



And the z value will stick around in the DMM file.

When you run the world, though, it won't actually change the location of the tile itself, and the tile's z value will of course, be wherever it's actually located. The world's maxz value will also be whatever it should be based on the size of the DMM file.

I actually found this useful for embedding excess metadata into a DMM file that allows me to do some interesting things when edited in an external editor...