+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.