Taxcode in Non Tax condition type in Billing Document

Hi

After saving billing document we see condition details in condition tab page of line item that non tax condition type like PR00 then we will find along with GL Account , Accounting key and find Tax Code.

Doubt

What is use of Tax Code for Non taxable condition type like PR00?

How system getting Tax Code for non taxable condition type like PR00.

Regards

Arun