thanks @HSheild and @SeanHenderson for the really thoughtful answers...i appreciate it.
Firstly, to confirm, I am using a CDS core schema environment....not Dynamics.
Can I just sum up where I am with my thinking and pose some questions:
1. I am somewhat confused about 'Regarding' (and I have raised a question on this previously and not really got to the bottom of it). If I generate an email, or an appointment, I would really expect the event to show on the Timelines of all addressees or invitees, driven by the 'To' field, or even the 'cc' or 'bcc' field. After all, the event does involve those people to them. But 'Regarding' allows only one item to be selected. What are your thoughts?
2. If I wanted to extend the filter of Timeline to include not just Regarding, but all those above, how would i do that?
3. There just seem to be too many different terms being used here for it to be clear. 'Timeline' suggests timeline of 'events' but it doesn't really clearly suggest what those events are. 'Regarding' is a clear field..though sadly allows only one entry. You then have 'Associated' and 'Related'. What do those two terms actually mean. Is associated that broader definition of To/cc/bcc etc
4. Finally comes 'Include Related=On' What does related mean...does it broaden it to not just the regarding and to/cc/bcc but also every item that was sent by the user, even if it doesn't relate to the Contact in question? That just seems counter intutitive that setting that to 'on' tightens the filter not loosens it. It perhaps would better say say 'includes related only', perhaps?
5. As for your point about Posts entity etc. My MDA has brought in Account, Contact and Activity entities, but I assume makes use of any necessary dependencies in the broader CDS core schema. Are we suggesting that this full activity functionality has dependencies outside of that schema that are therefore missing?
Thoughts?