On the target right around the boss is the inner circle. If you are behind him, then there is only one line between you and the boss (the inner circle). The outside half circle with the pointer telling you which way the boss faces is the start of the "flank" where the line starts on either side of the boss. You can slide a "couple virtual feet" by skirting the target ring where these two meet and get both your flank, and rear attack bonuses.

using a 12 hour clock as a reference:
With the boss facing 12
5-7 = rear
3-5 = flank
7-9 = flank

hope this makes sense.