The axisrms UV variable holds the tracking error (in arcsec, in Az and El) for each antenna in the array. It can be useful to automatically flag data when the tracking is above a certain error, or even antennae based (e.g. allow OVRO to have a smaller tolerance than the BIMA antennae).
% varplt vis=c0048.umon.1.miriad device=/xs yaxis=axisrms options=overlay yrange=0,100 % uvflag vis=c0048.umon.1.miriad 'select=-pointing(0,5)' flagval=flag options=noapply
The exact amount (5 arcsec in this example) is left to your own judgement, and you should probably also base this on the inspection of the graphical output of varplt. But in case you were wondering, the recommended value is 5.