This function is PROTECTED, and can only be called from Blizzard code. It has been protected since patch 2.0.1.

This function will select the nearest unit friendly towards you (i.e. is green if you would target it). It has nothing to do with whether you have added the unit to your friends list or not.

TargetNearestFriend({backwards})

Parameters

backwards 

Boolean : reverses the order of targetting units.