If the unit could not retreat and the combat result is dR the unit take one step loss and remain in place. If the unit could not retreat and the combat result is d1R the unit take two step losses and remain in place. One additional step loss to the step loss required by the combat table, if none is required just take one step loss and remain in place.
Sorry if you know this already, just to make sure we are on the same page: dR^R has two meanings depending if one unit is defending or if muliple units are defending. If one unit is defending the result means just dR, so the defending units retreats, if it can't receives one hit. If multiple units are defending. One unit suffers dR like before and all other defending units involved suffer a R. Each unit has to retreat, if it can't, it receives one hit. So it's always one additional hit.
Or does additional in this context has some special meaning to a native speaker, that I am not picking up?