Attribute rules – are user-defined rules that can be used to automatically populate attributes, restrict invalid edits, and perform quality assurance checks on existing features when they are submitted. Attribute rules are coming to hosted feature services and to forms so that you can execute business logic in your form connected or offline.
As @JayantaPoddar mentions, attribute rules are indeed on our road map for Field Maps. In addition, we do intend to support additional calculations that can be defined in the form itself.
