New Tax Classification Code

I have recently added new tax classification codes to ZX_INPUT_CLASSIFICATIONS and ZX_OUTPUT_CLASSIFICATIONS lookup types for use in tax rules.  The issue I'm having is that the newly added tax classification codes are not available to create rules with.  I have found notes referring to assigning the tax classification codes, but I am unaware of how to do this.  Does somebody know how I can use newly created tax classification rules in creating tax rules?


Thank you,
Elly