Once a unit is Earn-Unlocked, they can be added to an Earn formation.
Leaving units in an Earn formation will let them gather Hime Notes over time.
The Earn formation starts with 3 slots, but more will become available as units are Earn-Unlocked in that World.
4th slot: Earn-Unlock 2 units.
5th slot: Earn-Unlock 4 units.
6th slot: Earn-Unlock 6 units.
(If the Idle Meter is at 5% or greater, you will need to claim your rewards to change the formation.)
(Even if the number of Earn-Unlocked units decreases due to Scouting, etc., those units still count toward total Earn-Unlocks.)