Yes, you were right in your original post Vit. David is also correct. To notify for the very specific change in booking status from soft to hard, you would have to configure a custom process. We confirmed with our talented Support staff here. That is a very specific change. Typically, the notification template for requisitions handles its own set of notifications, but that change from soft to hard booking is a specific case, and you want to notify the actual resource I believe.
For documentation, users would enable notifications here (Set Up Notifications):
Personalize CA PPM - CA PPM SaaS - 14.3 - CA Technologies Documentation
And Resource Managers would want to consult this page:
Fill Resource Requisitions - CA PPM SaaS - 14.3 - CA Technologies Documentation
Choose your version and product edition, of course. Hope this helps. Thanks for asking.