How to link User Created Variable to PO/BPA attribute to use it as as a Rule

We are able to use System Variable to create a Rule.

For Example, Variable - Supplier. And based on XYZ Supplier on 123 PO/BPA we are able to apply ABC Clauses to that 123 PO/BPA .

But when we create a User Variable and use that to create a rule, then in that case Clauses are not getting applied to the PO/BPA.

For Example we have a Field in PO Line called Item. And based on the Item we need to apply the ABC Clause. In this scenario we do not have any System Variable for item, so we tried creating a user variable and use that user created variable to create a rule but that rule did not work as User Created Variable are not link to Item PO/BPA attribute.