+1 combat result for each extra rank with at least five models, to a maximum of +3.
The extra ranks of a unit's formation are not solely there to provide replacements to the fighting rank – they push the front rank forward and this momentum can swing a fight all by itself.
If your unit's formation is at least five models wide, you can claim a bonus of +1 combat result for each extra rank of five or more models behind the fighting rank, at the end of the fight, up to a maximum of +3. Note that this bonus can be claimed for an incomplete rear rank, as long as there are five models in it.
This is cumulative with several units engaged in the same close combat, calculating ranks for each unit, as long as they are engaged in different arcs of the enemy unit (up to a maximum of +3 per arc). If two units engage an enemy unit in the same arc, count only the highest number of ranks from one of the units.