Sometimes a revision is a copy from an external page that is freely licensed, but still require attribution. It should be possible to add this in a structured way, also after the revision is saved. This could be some way of annotations for the revision, but it could also be done by a simple interactive logging facility. It should not be a tag, that is more of a temporal tool. However this is implemented it should be listed as part of the history page, so attribution of the external page is clearly stated together with other credits for the page.
I am not sure, but I wonder if this is a type of annotation, and that the types should be a limited set where one entry is "provenance".
Also note the difference between annotating a specific revision (the change) and annotating the rendered content.