The function appears to work in two dimensions, and as such can be used to find the position of a target relative to the player if bearing and distance are known.
However, if there is a significant change in elevation between yourself and the target, the distance registered on a rangefinder (which is measuring in a 3-dimensional straight line) will always be larger than the two-dimensional value required for BIS_fnc_relPos and as such air-strikes and ordnance called in using such a function can miss by several hundred metres in extreme cases.
Currently searching for a method which incorporates the azimuth of the rangefinder in calculation.