Moveable entities need a collision model (cm) or the map won't run. This is a defined shape simplified to help performance. In particular, if you create one of the moveable custom entities you need to consider this.
This can be built into the model file itself.
A separate cm file can be created in the editor (see Moveables) eg, name.cm.
The entity can reference another model file's cm using the clipmodel spawnarg. In particular, some models have had a separate model file created containing only a cm and these are in models\darkmod\misc\clipmodels. For example, barrel01_cm.lwo.
- Models - Overview over the different kind of models