I found this on one forum. Should be helpfull.
When your subclass is level 75 depending on what arch type it is you will only be able to pick class skills based on that archtype. Eg. If you level a Titan to 75 when you talk to the npc and you "redeem" your subclass for the new bonuses you can only pick from Warrior Passive and trigger skills.
Warrior: Duelist, Grand Khavatari, Dreadnought, Fortune Seeker, Maestro, Titan, Soul Breaker, Berserker
Rogue: Moonlight/Ghost Sentinel, Sagittarius, Wind/Ghost Rider, Adventurer, Arbelestor
Knight: Phoenix Knight, Hell Knight, Shillean/Eva's Templar
Summoner: Arcana Lord, Elemental/Spectral Master
Nuker: Storm Screamer, Mystic Muse, Archmage, Soul Taker
Healer: Cardinal, Eva/Shillen Saint
Buffer: Spectral Dancer, Sword Muse, Heirophant, Doom Cryer, Dominator, Inspector
EDIT 1 - Additionally all Chance skills occur when the player attacks a target, all Counter skills occur when the player gets attacked by others. In both cases the chance/counter effect creates a buff that appears on the players buff bar. The buff is short lived typically around 10-30 seconds depending on what the skill is.
EDIT 2 - Passive and Enhanced Passives are Stackable. L75 Archtype Passives and Trigger skills do not stack.
EDIT 3 - Any skill can be untrained for 10,000,000 adena at the Ivory Tower NPC.