I dont know about a definite link to some official information, but here is what I gathered. [quote]Toughness: Increase max health Speed: Increase movement speed and jump height Recovery: Increase speed of regen after regeneration kicks in[/quote] One of these is prominet in the original setup of each subclass that exemplifies the class it belongs to. Titans have high toughness, this was the reason they are considered harder to kill (using focus upgrades you can get a warlock to beat a titan in toughness. It just comes at a higher cost to speed and recovery then it would for a titan to increase toughness), Hunters have high speed, and Warlocks have high recovery.