What does it mean when tax lines on a PO matched invoice are marked as 'Manually Entered' in Tax Det

Hello -

Can anyone provide some details regarding the 'Manually Entered' check box on the Tax Lines Summary and Detail Tax Lines forms in Payables?   Specifically this scenario:  Invoice is matched to PO, click 'Calculate Tax', Tax Lines Summary form shows the tax as 'Manually Entered'.  User cannot override.

Any help would be appreciated!!

thanks,

Sharla Skerbinc