GST PH-II patch applied. While doing TDS Setup-->Common configuration-->Withholding tax tab -->tax c

GST PH-II patch applied. While doing TDS Setup-

Under Common configuration-->Withholding tax tab -->tax category is to be provided only when the registration number is not available .   Kindly clarify whose registration number  is not available ?

-- First party, third party or any other ?? 

When should this common configuration setup be done ?  For all tax categories  or for some exceptions only ??

Kindly clarify.