It's good to see you here in the Community space, chessus.
The Permit no. on the Purchase order shows when you've set up multiple sales taxes. And the only way to remove it is to turn off your sales tax. However, we don't recommend turning off this feature as it may cause problems in tracking your sales taxes.
I have some resources that might come in handy. These links will guide you in determining the tax exemption status of a transaction or customer, and also assist in assigning a sales tax to different items and services: