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Spawned Species

From Diablo 2 Wiki

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