I think it's worth noting that the Warhammer 40k community lacks a standard, systematic way for talking about the features of the game. Everyone has their own jargon, and that isolation. I like to think of building an army able to maximize all three of the following:
Synergy (special rules synergy, basic rules synergy)
Flexibility (anti-infantry, anti-armour, anti-air; longer range, more units addressed, more actions to do)
Redundancy (# of weapons, # models, #units; lower cost tends to mean greater redundancy)