Hi @him2 ,
You can utilize <check> <error> tags for customized error popup
<input xpath="@email" required="true"/>
<leave>
<check expr="@email!=''">
<error>The email address is required.</error>
</check>
</leave>
The above script will throw error popup on input form, if Email ID is empty.
Similar way, you can make expression in <check> based on your requirement with product and sub-product, and can customize the error popup.
For more details on this, Refer this document