A surf, or .srf file is a multi-use file, used for low detail visuals, collision files, cockpit files, and the building blocks of the visual .dnm files.
Each .srf file starts with a line with "SURF"
Vertex Section Edit
The first line of the code declares the file is, in fact, a surf file. Below that lies all the vertex information, giving the vertex's position on the X, Y, and Z axes:
V 0.286748 0.958462 5.806995
Polygon Section Edit
The polygons are then defined by the line number of the vertex. For example, the polygon below has four corners, vertices 1, 2, 25, and 26.
The colour is defined in the C line by an RGB code. This polygon is a medium grey. The polygon is then ended with the E.
V 25 26 2 1 N 0.153079 0.697158 5.892211 -0.309806 -0 -0.9508 C 127 127 127 E
If the face is to be set as bright, there will be a line in the face definition that is simply "B":
V 25 26 6 1 B N 0.153079 0.697158 5.892211 -0.309806 -0 -0.9508 C 127 127 127 E
The last few lines of the .srf can define it as a transparent object (ZA), or a light (ZL). Setting it as a light increases the brightness and decreases how much it darkens at night.