On the Tax Override Program (P060120) under the Additional/Flat Tax Column What Are Codes X and Y us

On the Tax Override Program (P060120) under the Additional/Flat Tax Column there are 5 codes in the UDC table %, A, F X and Y.  We read the glossary for the codes and tested changing the field to either X or Y and performing an interim, but there was no change in the tax.  Are these two codes active and how are they used and when would you use them.

Thank you,

Kathy