Odoo 14.0 (enterprise)

How to create or design an activity type that, once marked as done, returns a notification to the original sender.

Case:

- User X wants User Y to review a document (sale order, invoice, whatever)

- User X creates an activity on the document and assigns it to User Y

- User Y checks the document (how or what is irrelevant) and responds with "OK" + marks activity as done

-> How do you notify User X that User Y has performed the action?