How was the value of the "useme" flag in the NOMAD astrometric catalog set? The readme just describes the field as "Recommended astrometric standard," but says nothing about the basis on which that recommendation was made.
It seem to be a difficult thing to track down, but perhaps the recommended catalog descriptions on the USNO site can offer some explanation. I couldn't find something that directly gave the methods used to determine whether or not to set the
useme flag, but looking at the NOMAD recommended ordering of different catalog data, as well as the catalog descriptions themselves, might give more insight on this.