Understanding of terrain .att files

Hello!

I'm trying to understand the format of terrain files. Right now, my understanding is that there are three kinds of such formats, all of which can be identified by their file size: a server format and two client formats.

They all boil down to an attribute map. The part I'm having trouble understanding is the values of these attributes. From the decompilations of GS it seems like the possible values are 1, 2, 4 and 8 (and sums of them) and that 2, 4, 8 mean that the terrain is unpassable. However, I don't know the exact distinction between them. Could anyone clarify what each value means?

Thanks!