Components are required to build and promote your units. They are earned through certain Command Center Crates depending on rarity or from the Black Market. There are only 2 types of components available; Common and Critical.
Earned through footlockers and briefcases of any rarity. There are only 5 types of Common components reflected by the unit type. They are only used for promoting.
Earned through silver footlockers, golden briefcases, and stronghold crates. Each and every unit has their own critical component. Building advanced units like the Machine Gunner, Abrams, and more actually require their own components in order to do so, indicated at the top of the metal cost. Advanced infantries only require 10 critical components while others require 20.