Where the Federation Space Force is concerned, all mobile suits are marked according to their mothership. Katoki (and the Master Archive book) uses two-letter abbreviations, such as "WB" for White Base, "AL" for Albion, "MR" for Mariana, etc. The recent MSV-R series tends to use hull numbers instead - SBB-17 for the GM Cannons stationed aboard the Luzal, SVC-71 for the GM Intercept Customs assigned to the White Base II - but it's the same concept. That's a small enough group that they only need a couple of numbers to tell the mobile suits apart, and when they're using three digits, the first one is typically the team number and the second is normally zero.
Thus, Katoki's interpretation of the RX-78 Gundam is marked "WB 102" because it's the second machine of the White Base's one and only mobile suit team. The machines of South Burning's "Immortal 4th Team" are labeled "LV 401" through "LV 404", which might mean their mothership carries multiple teams, or perhaps that it operates as part of a larger squadron where they need to be able to distinguish the teams...
