thanks for the response Peter - I've done some more doodling, and can now answer my own question as well. At the IPTC website http://www.iptc.org/IPTC4XMP/
the downlodable pdf Schema for XMP goes through step by step each section. It's an interesting read.
page 9 - The IPTC content panel - "The Description field, often referred to as a “caption” is used to describe..............."
While you're at it; the last section of that speci describes how the "old" IPTC fields are matched with IPTC4XMP properties. There you'll see that the Caption field maps to the dc:description field.
Title would be even shorter. I personally use the title field to store a backup of the filename, in case it gets changed by a client later on down the line.
Peter, that is the way to do it. Unlike the misleading name of that field (dc:Title) the field is intended to be filled with *your* name of the file. Or as the specs call it:
"The title of the document, or the name given to the resource. Typically, it will be a name by which the resource is formally known"
Unfortunately many people use this field for a short description; which should in fact be entered in Headline (photoshop:Headline).