the process of concentrating on and becoming expert in a particular subject or skill.
"the division and specialization of labor"
Which is to full health bar by using holy spells because there's no active discipline spells worth using. The entire tree helps the holy play style more than anything else. People didn't even ask if priests were disc or holy, they asked healer or shadow and if he said heal he asked if he had spirit buff. Cuz everyone knew it made no difference otherwise, you look at them playing in your raid and both the guys would do the exact same spells at the exact same time. There's a reason why the disc page got insanely change later on: because it was not a spec.