Enforce Tax From Reference Document - Clarifications required!

As per Note 735991.1,

Enforce Tax from reference document causes the system to use the same tax on the invoice that was used on the purchase order.  If your tax attributes change and you do not wish to force this validation then you should be sure to uncheck this checkbox.  Consult your tax department to determine when your tax obligation is incurred (at commitment or invoicing).  Many customers choose to disable this option.

In the same note, it is also observed that

The eBTax: Allow Override of Tax Recovery Rate profile option controls which users can enter or update the calculated tax recovery rates on the transaction for the tax recovery rate setups that allow this update.