XMP
XMP is a block written inside a file that says what the file is: its title, who made it, when, and which rules it follows.
What the word means
Open a document and you see the page. Beside the page, and never shown, the file carries a short description of itself. That description is what XMP is.
It is written as a list of names and values, in a form a machine reads without laying the page out at all. Title. Author. Date made. Date last changed. The rule it claims to follow. A search engine, a document store or an archive reads that block and files the document without anybody looking at it.
What it does on a real document
An archival document has to carry one, and what it says has to agree with the rest of the file. A document claiming one date in its identity card and another elsewhere is refused by a checker.
You are not held to the names everybody uses. A firm can add its own — an order number, an internal reference — as long as the file also carries a short description of the names it invented, so a program that has never seen them can still tell what they are.
Where it comes from, and why
The form was created by Adobe in 2001, to give every one of its programs one way of describing a file, whatever the file was: a picture, a page, a video. Until then each format had its own little box of information, and nothing travelled between them.
It later became an international standard, numbered ISO 16684-1. It is built on an older way of writing facts about things, called RDF, which is why what it holds is a list of statements rather than a fixed form to fill in.
See it on a finished document
A register whose parts say where they came from An archival sheet carrying fields of its own The rules behind this word, and the texts they are read from