What is the purpose of Tax Code in Location form (R12)?

We are on 12.1.3 and we are testing RPC 2.  One of the changes is that the Location form (PERWSLOC) now has a field called Tax Code visible.  I tried putting in a tax code and testing what would happen when creating a requisition or an invoice using that location, but I didn't notice any difference in functionality.

We calculate tax based on ship-to location.  Some of our cities have 2 tax rates, one normal rate and one lower "non-RTA" rate.  Currently the tax rate defaults to the higher rate for both location codes because that is the only rate provided by Vertex.  We manually maintain the non-RTA rates for the applicable cities, and they have to be manually selected by AP after tax calculation.  I thought by selecting "non-RTA" from the Tax Code LOV that requisitions and invoices would default the lower tax rate.  But this is not the case.