A character fighting in close combat can always choose to allocate his attacks against enemy models in base-to-base contact, unlike ordinary troopers who may only do so if their opponents have different Weapon Skills and Strengths. A character's attacks can be divided up as the player wishes, as long as he makes it clear before rolling to hit.
If a player chooses to allocate attacks in this way the character can only inflict casualties on the models in base-to-base contact with him. It can be imagined that the character is deliberately looking to cut down those before him instead of rampaging through the press of fighting troops, dealing death wherever possible.
Diagram: Allocating Attacks. An Ork character and an Ork warrior are both in base-to-base contact with a Chaplain. The Chaplain is fighting both an enemy character and an ordinary warrior, and so can allocate all of his attacks against the warrior, or all of his attacks against the opposing character, or divide them between the two opponents as he wishes.