The term "ISO" refers to an exact archive copy of the data from a GameCube optical disc (NOD format). Technical details of the GameCube release include:
Because the ISO uses standard AFS archives (originally from CRI middleware), tools like AFS Explorer can replace car models, textures, and audio – though the GameCube’s memory limits will crash if texture sizes exceed 1024×1024 total per scene. Need For Speed Underground 2 Gamecube Iso Highly