Both reinforcements and sub squad are part of the Reserve, which are units that do not start on the field but can be placed on the field afterwards during the battle.
Reinforcements only require 50% of their usual cost, but they can only be played after Boost Time begins. Conversely, the Sub Squad can be played from the start of the battle, but retain their full base cost.
When building your squad, proper planning and understanding each of your officers will allow you to utilize their full potential!