Facing is an integral part of combat. Facing defines how well a character can observe, keep track of, and deal with objects and characters around him.
By default, a character can only Counter an opponent that he is facing directly.
When defending against opponents directly behind a character, the defender takes a -10 penalty to defense. If the defender is unaware of the opponent, the defense penalty is a -15 and the defender is Flat-footed.
Flanking is when two opponents are on opposite sides of a defender. When a defender is being flanked, he is automatically Flat-footed and incurs a -4 penalty to defense against all opponents.
A defender is considered surrounded when he is in mêlée with at least three opponents. Attackers who are surrounding a defender gain a +2 to attacks against that defender.