When certain units reach level 20 they may still have an empty slot for a potential. You can unlock that hidden potential by getting 5 points by completing the following tasks. Then, go to Castlefront Street and speak with Ellet. She will say that she interviewed a rising star from your squad and unlock the hidden potential.
-Participate in a battle (1 point)
-Kill an enemy unit, heal an ally, repair cover (for example, a sandbag) or tank (2 points)
-Capture a base (3 points)
-Save a fallen comrade (4 points)
Note: To get five points easily, enter Skirmish 1. Kill another allied unit, and collect their body to get 4 points for saving them. Select another unit and kill the unit that was previously killed. Repeat as desired.