Characteristics validations in BPS

I have a question related to characteristics validation in BPS.

Is it true that the validation is not executed if any one of the involved characteristics have unassigned value?

We have two characteristics: Account and “Trading partner” in the validation rule. We want to make sure the trading partner is assigned for certain inter company accounts. We have a validation rule (Exit type) that performs this check. But if the user did not input any value for the trading partner, the validation is not getting executed at all.

I could use enhancement SEMBPS01 to perform the same validation.

Thanks you for your comments and advice.