Why is the field Merchant Reference not available in Determining Factor Names

Summary:

For Expenses, I enabled the Expense Location and Merchant Reference fields to use in tax calculation. However, the Merchant Reference field is not available when I want to define a Determining Factor Set.

What setup is required to see the Merchant Reference in the list of values for Determining Factor Names?

Content (please ensure you mask any confidential information):

Example of enabled Expense Report Tax fields:

Available determining Factor Names:

Version (include the version you are using, if applicable):

24C

Code Snippet (add any code snippets that support your topic, if applicable):