Question

[Keydah]

I'm assuming off the wording you're talking about cross-class skills? If you aren't, ignore the rest of this.

The way it works is some but not all actions are cross-classable. When you don't have a soul stone (what turns you into your job at level 30), you can cross-class from any class's pool of cross-class skills. When you have your soul stone equipped, you can only pull from two classes that have abilities that fit it most.
For example, as an arcanist, you can cross-class flash from gladiator...there is no reason to EVER use it on arcanist (it purely generates aggro), but you need it when you play a marauder, so it's available from gladiator's cross-class pool. As a scholar (one of the arcanist jobs) you cannot use any gladiator cross-classes - you can pull from thaumaturge and conjurer only, because they have the most useful skills cross-classable. (stoneskin, cleric stance, and protect from conjurer, and swiftcast from thaumaturge)

Only actions that would be useful to other classes are available for cross-classing, so you can cross-class Physick to a Black Mage but not Shadow Flare (level 50 arcanist skill). Black Mage can also pull cross-classes from archer because archer has personal enmity-reducing (very very useful when you're nuking stuff for thousands of damage) & personal damage buffing abilities.