From TDN

The DML file format is used to define the textures used in the Torque skybox system. It is very simple with each line defining the name of particular texture. No comments or empty lines are allowed.

sky_back
sky_left
sky_front
sky_right
sky_up
sky_down
sky_environment_map
cloud_layer_1
cloud_layer_2
cloud_layer_3

The first 7 images make up the sides of the skybox itself and the environment map. They are required for the DML file to be valid. The remaining lines are for cloud layers and do not need to be present to create a valid DML file.

The textures themselves need to exist in the same folder as the DML file and must be power of two square. If necessary for saving disk space textures can be different sizes.