I went back to take a look at the data in SFFiles. It looked to me like the terrain descriptors were originally intended to be mixed in with other types of texture descriptors within the game. The descriptor field was 3 bytes long. I'm getting this from Appendix G of text.txt in SFFiles. Even if this system was not was eventually used for determining planetary terrain, it might give us a place to start.

On another topic, there's a list of source directory files in dir.txt within SFFiles. That information may give us the name of a file that was included in the final compilation of the game, which may indicate where the phrase construction part of the SF code is located. I'll get into the hex code and see if I can't find references to the communications files...I've spotted at least four possible candidates.