PMODEOFPUBLICATION is connected to WRITING > PRODUCTION. It deals with specific ways in which a book is brought to print; it does not address issues of circulation. It captures specific issues around the publisher (or details about the printer where pertinent and different from the publisher) through capturing particulars of specified types of publishing. It encloses full sentences, with enough information to convey the issues at stake. The attribute values on this tag cover limited, pirated, or private editions, publishing by subscription, and self-publishing. Other publishing issues call for other tags: PTYPEOFPRESS, PRELATIONSHIPWITHPUBLISHER, PEARNINGS, etc. This element has no mandatory or optional sub-elements.
A note on self-publication: this attribute doesn't refer only to those works published by vanity presses (or, today, independently on the web). In some cases, where the title page bears the words Printed [sometimes published] for the author, it means that the publisher wouldn't take the risk, so charged the author for publication and any profits then went to her. It was a sign of the author's confidence in her own work and could work out very well financially, or not.