How do I know which tax rules apply to calculating the tax as exempt when generating PO?

Summary:

The tax is calculated on the purchase order when in fact the tax should not be calculated because there is a rule stating which entities should be exempt.

How can we know which tax rule should be exempt?

Content (please ensure you mask any confidential information):

Within the tax configuration we have the following tax rules:

US_SPECIAL_RATES type of rule Tax Rate Rule

DRR_TRX_LINE_TYPE type of rule Direct Tax Rate Rule

How can we know which of these tax rules is the rule that applies the tax calculation as exempt when we generate transactions?