It is defined as
DL is derived from the cosmological redshift, assuming the following values for the Hubble constant and other cosmological parameters:
The choice of vvir to estimate the cosmological redshift is a compromise. For large distances, v3k should be more appropriate. But the galaxies of the Local Universe share a similar bulk motion as the Galaxy, and correcting only the motion of the Galaxy would bias their distance determination. An improvement would be to use a model for the bulk motions in the Local Universe.
The parameter mod0 (redshift-independent modulus) is an estimate of the luminosity-distance independently of the redshift, and both parameters are combined into modbest, to provide the best distance estimate at any redshift.