OFI: Third Party Registration (Customer-Exemptions Feature)

We have defined exemptions and setup exemptions for an customer in third party registration

So if any transaction done for the customer whether we will have exempt tax automatically derived/applied or do we need to apply the exempt tax category manually ? It seems it is not working tax category is not defaulting after above setup. We still need to setup tax determination rules to default tax category 'Exempt'

How we can use the exemption feature in OFI as document in info center is not clear.