How to change the variables for a tax component (in settings)

Just that. I need to change the variables for a tax component but it's blocked and no changes can be made.
How do I unblock this?

Why do I need to do this?

I need to create a conditional Sales Tax base in a field I added at: "admin/commerce/customer-profiles/types/billing/fields"

To reproduce:

1) I added a billing field called: "VAT NUMBER" at: "admin/commerce/customer-profiles/types/billing/fields"?

2) I created a Sale Tax at: admin/commerce/config/taxes

3) For this new Sale Tax I clicked at "configure component" to edit the "rule component" for the tax.

4) Now inside the rule: in Conditions I need to check if the field: "VAT NUMBER" is empty

Here is my problem: I don't know which is the Data Selector to get the field: "VAT NUMBER" and make the comparation.
I think that adding "Commerce Customer profile" data type as a variable to be used as input parameter (under the component settings) will help me.