For a particular website the client needs to link to both posts and to archive pages from a Relations field. How might I add a link to an archive page to a Relations field? The client is using a licensed copy of ACF Pro. Thank you.
That is not a feature of the relationship field. It is only a feature of the page link field. You could use a page link field and allow multiple selections, but the return values will be URLs and not any of the other alternatives available in the relationship field.