Child record registered on subgird cannot be used on parent record unless the user proceed development on their instance.
So, for example, if Child record primary key which registered subgrid is automatically stored in Option Set field, the customer-side development will become very simplified.
When the customer proceeds 1:N Relationships, search field is automatically added on N side.
Therefore, if such an Option Set field is also added on 1 side, it would be very convenient, extended performance, and high possibility of various usability for users.