Sap has been designed in such a way that few transactions mandatory require tax code as input for successful processing.
But since transaction is not tax relevant hence we don’t want any tax calculation to happen. In such scenarios, tax code with zero percentage tax is used e.g. A0 &V0.
For posting vendor invoice using T code: MIRO
Sap has made it mandatory to provide input tax code for posting vendor invoice using MIRO.
Hence, tax code V0 is used for non taxable transactions.
Configuration in sap can be done to take default tax code V0 whenever MIRO is used. If transaction is taxable, you can always change the tax code. If you don’t change tax code, then system will use default tax code V0 assuming transaction is non taxable.