If you look at UP System Timetable #7 that came out in 10-1989, up to and including that issue, it was the Oakland Subdivision. When UP System Timetable #8 was issued in 4-1991, the Oakland and Sacramento Subdivisions were combined into the Canyon Subdivision. When UP Switched to Area Timetables starting in 10-1998 (there were and still are 18 Area Timetables to cover the entire railroad) Robert E. Lee, who was in charge of putting these together (it took him 2 years and month after the Merger with SP to do it) decided that some of these Subdivisions, such as the Canyon were too long and large, so with the issuance of UP Roseville Area Timetable #1 in 10-1989, the separate Oakland, Sacramento, and Canyon Subdivisions were re-instated as such, just the way they were in UP System Timetable #7 from 9 years previous.
Also, with the issuance of UP System Timetable #8, the control point numbers and letters were established systemwide. The letter general stood for a service unit, in the case of "F", it stood for Feather River Service Unit. The number after it was usually the nearest milepost, but could be up to .9 miles away from it. "C" was California Service Unit. "G" was for Green River Service Unit. There were some that were different such as "I" was for the Illinois Service Unit, even though there was no such Service Unit, and no,"I" was not established for the Idaho Service Unit. It was a great idea at its time, but with all the additions/deletions/revisions of Service Units all the time, in the end, the letters were out of place. With the addition of former CNW territory in 10-1995, these existing single letters were put in place on those former territorys where there were no duplicate mileposts. For example, the U was used on the former Katy between Ft. Worth and Kansas City. When the Albert Lea, Mason City and Trenton Subdivisions, A.K.A. the former Rock Island were inherited from CNW, the U was extended here.
Finally, starting in 1999, and continuing until it was done, since almost all the letters of the alphabet for control points were just about used up, it was then decided to use two pairs of letters, and there was no guidelines for there designation at this point, however, "NI" is used for Niles Subdivsion, "AL" is for Alhambra Subdivision. "CO" is for Coast Line route from Oakland to LA. "SP" is, well, former Southern Pacific from Elvas in Sacramento, via Tehachapi and Cajon Pass to Tuscon.