Leslie,
is it possible to update just a collection with iptc info added to the original collection photos after the collection was added to a catalog? or do you have to do the whole catalog where the collection is?
When you change metadata on an image it affects all parts of the catalog if the image is in a catalog. It doesn't matter if the image is in a collection, just in a folder, part of a search, etc. If the image is known to a catalog and you change the metadata in Photo Mechanic Plus, the metadata on the file itself will be modified (or the XMP sidecar file) and the catalog will be updated.
-Kirk