Custom data rules for a table when a filed changes value

I assume Asset Class and Eval Group are separate tables and that you have Asset Class pulled into the Eval Group Table.

If that's the case you need to pull the Related Eval Group - Asset Class into your target table.  Then you can either do a formula field with form rules or table level custom data rules (use this if you have api's or pipelines that need to follow the same rules).

The data rule formula would be If([Related Asset Class]=[Eval Group - Related Asset Class], "error message")