We had to do our own crash course on this entity and how it works with Marketo over the last few months. There are no references in the Marketo documentation or model, however, we did learn a couple of things after syncing our database and then inspecting the model of those opportunities that were visible to Marketo.
- New Dynamics365 instances feature a stakeholder grid on the opportunity form. Creating contacts with this form actually creates external connections of type "Stakeholder" on the opportunity. These can be viewed in Marketo on contacts that have the "Contact Has Opportunity" relationship populated.
- Note that if the contact has not synced to Marketo, the associated Opportunity will not sync to Marketo.
Our org does a number of other things with Connections, but this was the primary place that we've been concerned with it related to Marketo. Happy to share more experiences if you want to reach out directly.