Back in ARR and HW, we use to have something like sub-jobs called Cross-Class Skills.
As you leveled a class (not job), some of your skills you gained could be used by other classes/jobs.
Classes could pick skills from any other class, while Jobs could only get skills from 2 specific classes.
For example: PLDs could get skills from MRD and CNJ, WHM could get skills from ACN and THM, etc.
You could only select a certain amount of skills to use though, gaining the option to pick more as you leveled, with 10 being the max at level 50.

They replaced them with the Role actions in 4.0 because it wasn't very newbie friendly and certain skills were mandatory, while others saw little-to-no use.