Before the expansion there was a system called "cross class".
There you could use some specific skills from classes you have leveld on other classes, for jobs the range of classes they can draw skills was very limited and more focused on their role.

Since the expansion the "cross class" system is removed and we got a "role skill" system.
There you will get skillpool apropiate to all classes/jobs of a specific role, from which you can draw skills. The skillspools are different for each role (tank, melee, ranged, magic and healer).

For example before the expansion as Paladin you could draw skills from Conjurer like Cure, Protect or Stoneskin, cause Conjurer was one of the specific cross classes for Paladin.
Now Paladin can only draw from the role skill pool for tanks, which are including vital skills like Provoke, stuns, silences or simple defensive cooldowns.