Where is flank?

[ChaozK]

When you target a mob it puts a circle on him. The rear is indicated by a small missing area of the circle. The flank is just next to that.

[Lgalang]

For the above example picture the flank is the bar on the side of the target. the pointed arrow is the front and the back is the one missing behind. so your right the 7/5 oclock position are still considered flank. and the 11:30 and 12:30 position are considered back.

[Houston009]

If you look closely, there is a secondary half-circle near the front of the target circle, that indicates the front. So anything between that half-circle and the back is considered the flank.