The usual first estimate scales the gamma ray between a clean reading and a pure-shale reading. Neutron-density separation is used as a check, because a radioactive clean sand fools the gamma ray. The result is written Vsh.
Clay volume, the fraction that is clay mineral, is smaller than shale volume, because shale includes silt and water as well as clay. Using a shale volume as if it were a clay volume over-corrects the porosity and the saturation. A cutoff on shale volume is one of the choices inside net pay.