Generally speaking (and there will be many that can clarify my points and correct any mistakes I happen to make [it's late here]) for North America:
The Great Ghost Dance and resulting war splits the borders between Native and Non-Native countries. Internal to the NAN, other than roughly being the locations for the tribes that lived there...I have no idea as to the rhyme or reason to the divisions other than: SE PPC's border is that of western Texas and Oklahoma, N PPC's border with S-S is mostly the Snake River and other such obvious portions of the NAN borders. But portions like Sioux-PCC border, especially north from Denver to S-S border...
Most of the Sioux-UCAS border follows either river routes or county borders, though there are some exceptions where counties are cut in half. So to me this indicates where the military standstill by 2018 occurred...but I don't know of anything canon that fully supports that.
If I remember correctly; Aztlan originally increased their borders into AZ, and NM, in 2018 but then later invaded CA and Texas (in the 30s?). So Aztlan CA/AZ/NM borders with PCC would fall into the 2018 treaty vagueness of NAN border decisions. The Azttech...er Aztlan/Texas border is a result of invasion...where the war cooled to a simmer.
Carib-League/CAS...best estimates I have but the border at the Everglades Parkway (75), for the majority of the border. I'm not in any way an expert on Florida history/demographics...but a quick check on pg 170 of the Sixth World Almanac and there's three sentences at the end of the first main text-paragraph that answer the Miami spit from CAS in 2034 (12 days after UCAS and CAS sign the Treaty of Richmond which formally recognized CAS as a sovereign nation). It's not an in-depth explanation, but it's there.
Part of the reason why there isn't a detailed outline of how the borders were drawn is (my main hypothesis) because it would be a long boring read that would be very tedious and time-consuming to write.
Take for example the S-S / PCC border. It would read like
"The corner of S. Roosevelt St and W. Overland Road, in Boise, marks the tri-corner mark of Tir Tairngire, Salish-Shidhe, and the PCC. The S-S / PCC border then moves east along W. Overland Road until S. Vista Ave where it turns northward and then follows S. Capital Blvd. In the middle of the Boise River, on Capital Blvd Bridge, the border turns east, following the river until the Diversion Dam where then then travels, at 139 degrees from north, 45 miles southeast to some sage-brush covered spot of barren land where it then heads northeast for..."
Take a look at the Google Earth maps in the GM's Toolbox sub-forums, read the histories, and then anything that isn't canon is free for the GM to make up the reason as to why the border is that way in order to further what ever nefarious plot they have cooked up.
