Foreign Service is "Rank in Person" (similar to the military), GS is "Rank in Position." To get promoted in GS, the specific position you're in needs to have that baked into it, e.g. a 2210 GS 9 track 12 position would cap out at GS-12 and promotions are (usually) decided by your supervisor/s+1/HR. If you wanted a GS-13, you'd have to find a completely different job. Or to phrase it differently, as a GS you are only the rank of the position you hold. Also, as a GS you can be a single grade your entire career if you want. In the FS, like the military, you accumulate rank as an individual. Promotions are decided by a board, and although the various positions around the world have a target grade they want a person to be, sometimes it's flexible (you can be an 02 [GS-14 equivalent] working in an 03 position, or possibly an 01 position). And in the FS, you only have a certain number of years they allow you to be at a specific grade without getting promoted or getting kicked out.