My impression is that they are randomly distributed sites, but random within a certain set of site types. I have not tested this, but it would appear to insure that the given province will contain at least one site out of the specified theme or grouping. I think I understand what you were saying though.
On a side note (for anyone interested) the bit math is really simple.
If you have terrain of plains (0) and want to flag it as containing a fire site (because you named it "The burning Plains"), then:
#terrain prvnbr 8192
0 + 8192 = 8192
To make that same province "nostart",
0 + 8192 + 512 = 8704
You can easily "decompile" it by simply looking at the list of values (that Gandalf kindly posted above
) and finding the largest value that will go into your terrain value and subtract it. Rinse and repeat until you have your complete list of terrain flags.
Pulled this terrain value from a random map:
524418
- "naturesite" 524288
= 130
- "forest" 128
= "large" 2
So the given province is a large forest province containing a random nature site.
(Sorry Corwin, I guess all of this really doesn't help you in your quest for modifiers.)