There are fancy codes, but you don't actually need to know them. Basically they map to the highest subgrade you are currently expected to be promoted to.

GEO5 (default)
SEO1 (have been recognised as having potential to go further; required for SH/LH/ST appointment - and I think HQ posting?)

Beyond that, it's pretty much just increasing step by step... SEO2, SEO3, Superscale H, etc...

(additional complexity - CEP also covers tracks, i.e. Teaching/Leadership/Specialist tracks, so it's possible to be recognised as a potential ST even if you're not so great at administration. But for the most part, the above should give you a rough idea of how CEP works.)