The object of the Add is a Collection, which would usually mean to add the collection obviously

You can introduce a new activity type, which will have different semantics.

e.g. DeliverBatch

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startTime: The creation time of the chronologically first activity in this batch.

I suggest clarifying what "creation time" means. Is it the value of the published property of an activity?

Delivery

The Add activity in the example does not have the id, actor and to properties, which are usually required.

Processing

Should receiving servers add Add activities to inbox collections?

@technical-discussion