At first glance the cut seems arbitrary. If the thought is that the YSO is heating a region around it, leading to bigger velocity dispersions, the size of that region should be dictated by something like the physics of a Stromgren sphere, and have nothing to do with the beam size. A similar comment applies to the case where the region is heated by a stellar wind or outflow.
However, if the beam has width 6”, and we assume that represents the full-width at half maximum, then 1 sigma is 3”, and 12” represents 2-sigma on either side of the Gaussian beam profile. So, while technically structure >6” would be resolved, anything within 12” of the beam will be smoothed by a Gaussian profile. Hence there is potential for contamination of the black histogram by the YSO if one is closer to it than 12”.
ISM and Star Formation 