I want users of my app to be able to write a "description" or "narrative" for a given picture.
I was primarily looking at the XMP section. Can someone advise me as to the most commonly used tag?
Use Description
Take a look at the IPTC Photo Metadata Standard 2019.1 (https://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata) spec for ideas as to what tags are supposed to be used for what purpose. It's a dry read, but the description, help text, and user notes for each tag give the basic idea of a tag's purpose. Then below, it gives you the XMP and IPTC IIM tag name, though you might have to look at the XMP tags page (https://exiftool.org/TagNames/XMP.html) or the IPTC tags page (https://exiftool.org/TagNames/IPTC.html) to get the exact exiftool name.
For example, for Copyright Notice (https://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata#copyright-notice), it says
(https://i.imgur.com/i2BcAfp.png)
In this case, you would use Country (or more specifically XMP:Country or XMP-Photoshop:Country) for the XMP tag and Country-PrimaryLocationName (or IPTC:Country-PrimaryLocationName) for IPTC IIM.