Max Rank:
If Slot Rank limits boosting to where the slot is, Max Rank endows each of your units additional boosts on their individual HP, damage, fire-rate and coverage by upgrading their ranks. After earning these passive upgrades, try to combine them slot rank boost to achieve each of your unit's full potential.
Max Rank 2:
Increases maximum recruit’s rank to 2
Max Rank 3:
Increases maximum recruit’s rank to 3
Max Rank 4:
Increases maximum recruit’s rank to 4