Class
HCF servers usually have 5 Classes. Diamond, Archer, Bard, Rogue and Miner. The latter of which is not used for Combat.
Activation
Every class has a specific armor set that you must wear to activate it. (Leather for Archer, Gold for Bard, etc). This makes it so you can basically always decide what class you'll play. Even in the middle of a fight, if you're in Gold Armor (Bard Class), but have a spare leather set (Archer Class) in your inventory. You can swap to Archer.