I was able to create some custom fields in the entity ActivityPointer (some web sites told me that this is not possible - but you can simply add them in Dynamics 365). Unfortunately these are not visible at the "inherited" entities Task, Appointment, etc. Even more strange: I can create fields with the same schema name in in activity entities like Task. I crates a custom "whole number" field called "sem_rank" in Task and later I created a whole number "sem_rank" in ActivityPointer - no warning, they seem to exist side by side.
Is there somewhere a description, how Microsoft thinks that we should work with custom fields in activity entities? I have some information that I need across all activity entities (even future once or such of other solutions) - and I need to show that information on the form for tasks, appointments, etc AND in the overview of Activities (which is not possible with fields added to individual activity entities). If Microsoft thinks of "ActivityPointers" and activity entities like "Task" as distinct tables, I will need to implement the same fields not only for each of the activity-entities, I also need to customize each and every additional activity entity provided by a custom solution and I need to implement "something" (like Azure Functions called via WebHooks) to write the values of these fields to the ActivityPointer entity ... would be a sub optimal design approach in my opinion.