Given the way that subfind defines subhalos, it is not obvious to me what the analogous choice of delta vir is; e.g. that the subfind masses are equivalent to M200c. It looks like some of the first Illustris papers make this assumption but I am having trouble finding an argument for why. Could anyone share some perspective on how best to treat the subhalo masses, especially in comparison to other works that use spherical overdensity criteria?
Subhalo masses are very physical, as they represent mass "gravitationally bound" to the object. This is not directly comparable to any SO quantity.
For every central subhalo, we would typically refer to its e.g. M200c as the Group_M_Crit200 value of its parent halo.
For DM mass I've just learned that the mass formation history for the FoF is practically identical to the central subhalo scale, but the FoF mass is larger. For stellar, gas or black hole mass, only the subfind mass exists. However the mass is only analogous to the central subhalo and not the satellite halos.