and what makes the difference between a class, and a class-option then?
to me…
a class-option has shared skills/abilities that continue to advance, as well as specialized skills/abilities that the other class-options don’t get.
–eg, after x-many levels, will you continue advancing as X or Y? (or other choices.) your new choice provides new options, but you can also continue advancing in the old one. like Ragnarok Online: you advance your job to 40 or 50, then pick a new one. if you start as a Thief, you advance to Assassin or Rogue. you can still spend points on your Thief skills, though, even after advancing… and some skills will require having a pre-requisite from Thief before they unlock.
a class would have separate skills/abilities that advance independently of other classes.
–eg, like ToS. every 15 job levels, you pick a new class to advance in. if you pick the same one, you advance to higher levels of skill. if you pick a new one, you stop advancing in the old skills entirely. Skill points you earn as a Priest in ToS simply -cannot- be spent on your Cleric skills. Access to Priest skills has no reliance on what skills you have or have not taken as Cleric.
TL;DR, to me:
a new Class means your old advancement stops, and a new line of advancement starts.
a Class-Option means your advancement continues, just in a somewhat adjusted direction.