XMP is an Adobe initiative to promote a standard for metadata, but it's not a proprietary initiative. Instead, it's an open standard, it's documented, it's extensible, and it's even somewhat readable by humans. It is, in fact, a subset of XML (Extensible Markup Language), which is turn a subset of SGML (Standard Generalized Markup Language), the international standard metalanguage for text markup systems recorded in ISO 8879.
If you want to delve deeply into XMP, I suggest you start by looking at the available documentation. You can find several useful documents, including one on building custom File Info panels, at www.adobe.com:80/products/xmp/main.html.
Because XMP is still relatively new, you'll almost certainly encounter some growing pains if you try to work with a mixture of applications, some that support XMP, and others that as yet do not. There are two things you can do to lessen, if not eliminate, the pain.
The first is up to you. The second is the core topic of this chapter. The metadata that you enter in Bridge for your raw files will persist through all the converted images that you create from the raw files, unless you take deliberate steps to remove it. This is mostly a huge advantage to photographersyou can enter the information once, for the raw file, and know that it will be present in all the variants that you create from that raw file, not as a sidecar file (those are only necessary with read-only raws) but embedded directly in the .tif, .psd, .jpg, or .eps image.
You know that your copyright notice will be embedded in the image, and, even better, you know that if you deliver the image on read-only media like CD-ROM or DVD-ROM, you can prove willful violation of the Digital Millennium Copyright Act of 1998 should someone else remove it. However, you may not always want to provide your clients with all that metadata. Some benighted souls still have attitude when it comes to digital capture: it's doubtful that they could identify the source of the image from the pixels, but they can do so easily from the metadata.
Metadata may seem mysterious at first, but with only minimal effort, you can gain a great deal more control over it. And if you're willing to do some serious heavy lifting, you can accomplish magic!
XMP Is Text
The first important thing to learn is that .xmp files are simply text files, readable by any text editor or word processor, that conform to a specific syntax and are saved with a .xmp extension. So it's easy to read and, if necessary, edit XMP metadata.
The second important thing to learn is how the user interface in Camera Raw and Bridge relates to the .xmp files that get stored in various locations on your computer. When you apply keywords or copyright notices, where does that data actually get stored? The answers may surprise you, but if you're at all curious, it's highly instructive to take a peek at sidecar .xmp files, saved Camera Raw Settings and Settings Subsets, and Metadata Templates with a text editor.
For the truly motivated, the third lesson involves the things you can do by customizing .xmp files. For example, when you save a custom Metadata Template, you may be surprised to see all the junk that by default gets saved in it. Judicious pruning with a text editor can make these important files more reliable. If you're really gung-ho, you can actually use XMP to make your own Custom File Info Panels. The IPTC schema, for example, is very much tailored to the needs of editorial shooters. You can use Custom File Info Panels to make it easy to add metadata to your images that more closely suits your own needs.