Spawned Species

In Levels.txt,

NumMon
Number of different Monster Types that will be present in this area, the maximum is 13. You can have up to 13 different monster types at a time in Nightmare and Hell difficulties, selected randomly from nmon1-nmon25. In Normal difficulty you can have up to 13 normal monster types selected randomly from mon1-mon25, and the same number of champion and unique types selected randomly from umon1-umon25.

mon1-mon25
Normal only. Spawned species, use ID from MonStats.txt. Only the first ten columns appear in the unmodded file. In 1.10 final, beta 1.10s and v1.11+ you can add the missing mon11-mon25 columns.

rangedspawn
Give preference to monsters set to ranged=1 in MonStats.txt on Nightmare and Hell difficulties when picking something to spawn. Exact way this is done is unknown though.

nmon1-nmon25
Nightmare/Hell only. Spawned species, use ID from MonStats.txt. Only the first ten columns appear in the unmodded file. In 1.10 final, beta 1.10s and v1.11+ you can add the missing nmon11-nmon25 columns. Due to what appears to be a bug Champion/Unique species uses this column also in nightmare/hell.

umon1-umon25
Normal only. Champion/Unique species, use ID from MonStats.txt. Only the first ten columns appear in the unmodded file. In 1.10 final, beta 1.10s and v1.11+ you can add the missing umon11-umon25 columns.

Source: http://phrozenkeep.hugelaser.com/index.php?ind=reviews&op=entry_view&iden=301