There's more than I thought.
EDITED
Some provinceIds have a number greater than the number of provinces. These only appear in the three bigger lightmaps, and they help the game to draw country borders.
The provinceIds are in 16 bits. The 15th is, like normal provinces, a signal as to whether or not there are more provinceIds.
The 14th is on to signal that this is a special border province
The 13-8th form an index. If you subtract 4 from it, you can go into the provinceId list and get a normal province number, which is one of the provinces that this border borders.
7 to 5 are always 0, as far as I can see, so I don't care what they mean
The 4th to 1 is another index, into the Adjacent.tbl for the first province. The result is another province that this border borders.
0 Is whether the province is colored by Outline or BorderOutline. To see this, look at a country border, there is redness, but also blackness on the border.
(May the Great Johan please tell us if something is incorrect?)
BTW, what did Jebus do? Did he manage to figure out everything about these maps? Is his information still available?