Sabah's modern S system is the product of several rounds of housekeeping.

Earlier North Borneo and Sabah formats experimented with different letters and positions. As overlaps and inconsistencies built up, JPJ settled the matter by standardising on S with distinct second letters for different regions—SA, SB, SD, ST and others—mirroring Sarawak's approach while keeping its own identity.

It is a rational end-state: S at the front, area encoded next, plenty of headroom. The steps it took to get there give today's prefixes a sense of being refined, not accidental.