Determination of Vc when dhecking AISC 341 Section 9.3a Panel zone shear.

Panel Zone shear Vpz is determined.  It is reduced by "Vc".  I can find no discussion of this variable in any manual or in any post on this site.  I have made a few attempts to recreate this assuming that it is shear in the column, but have had little luck in finding the load combinations which do so.  Where is a discussion of "Vc" which will adequate describe this value?