From CoDWWModWiki

Example: If the "mods" folder contains

     customMaps\

      hello\

            mod.arena   //It will overwrite the entire maplist when you launch hello mod

     newworld\

General rules

   * Fastfile(map name) in the map list should skip mod.ff, ui(_mp).ff, code_post_gfx(_mp).ff and common(_mp.ff)
   * We only accept two folders: mods and usermaps in Document and Settings\<username>\local settings\Application Data\Activision\codwaw\
   * No mods or usermaps should be kept in game installer directory, if you have them there, the launch mods and map names should not display anything there.
   * SP exe mod list shouldn’t list mp_
   * MP exe mod list should only list mp_
   * SP exe mapname list shouldn’t list mp_ folder’s ff
   * MP exe mapname list should only list mp folder’s ff
   * There should be no duplication in maplist

Explanation: If there are duplicates, like you have mods\hello\ hello.ff and usermaps\hello\hello.ff, in maplist, you should only see ONE “hello” map.

And if you load the map, it should use the mods\hello\hello.ff instead of usermaps\hello.ff

MOD.ARENA File

   * mod.arena file is a loose file now. Don’t put in the iwd 
   * mod.arena in \usermaps\<modname> should be INGNORED
   * mod.arena in \mods or \usermaps root directory will be ignored
   * mod.arena in \mods\<launchedMods> will REPLACE production maps. It doesn’t list any \usermaps\subdir\ff or mods\<modsname>ffs
   * mod.arena file if has same long names more than once, only show up one.

MODS WITHOUT MOD.ARENA File

• If you launch <customMaps> or <newworld>, since they don't have mod.arena, you should see:

1) Regular Production maps
2) All the ff files in usermaps\<subdirectory>\<subdirectory>.ff
3) Only the level ff file with SAME name as folder name in mods\<launchedMod>\

FF files Searching hierarchy

If ff file exists in zone/English, it has the highest priority

Otherwise, WHEN in Mods launched condition, search path would be

    */mods/
    */usermaps/