While specifying tax amounts on purchase order is not a recommended best practice it is common, especially in manufacturing industry, that customers may want to indicate whether an item line on PO is taxable and have that flag sent to supplier on PO document. In those cases customer will set up "Taxable/Tax-Exempt" flags via custom fields on one of below master data objects and then default the value to requisition/purchase order:
If needed the default can be changed by buyer during PO creation process. PO liquid template can then be customized to include the flag either at header or line level to communicate to supplier whether purchases are taxable on sent PO.
Taxable flag can be defaulted from purchase order to invoice in order to ensure that sales and use tax is calculated when appropriate. Flag can be included in outbound invoice integration to be used for any downstream tax processes in ERP. For instance if an item is marked as 'non-taxable' or 'tax-exempt' AP may need to calculate and apply use tax in ERP based on registered location of supplier vs. where item was shipped and/or request tax exemption certificate. This information can also be useful to check against whether supplier had entered sales tax to reconcile any invoices where there is discrepancy between taxable flag value and sales tax billed on invoice.
*Please note that above practices apply to customers with operations specific to United States