Set back is defined, where one of the front wheels is sitting back (relative to the wheel on the other side).
Negative setback is the left wheel back (relative to the right)
Positive setback is the right wheel back (relative to the left)
Set back would be identified when measuring the SAI and caster angles as the values would be too low and out of specification. Which side is set back or set forward can be established by reviewing which wheels castor angle is within specification.