Descriptive Metadata for Television Engineers [SMPTE] unique material identifier [UMID]). These are normally free. Given a registered identifier, it is a relatively trivial task to trace back through the register and find out when it was issued, what for, and to whom. With unregis- tered identifiers, this is clearly far from trivial, if possible at all. For example, given only an ISBN of "0-14-044107-7," most people would know it identifies a book; a bookshop can quickly identify both it and the publisher and order a copy. Given only a UUID of "6fdf4290-ae8b-11d9-9669-0800200c9a66" there is little chance of finding out what it identifies, though it is possible to extract some detail such as the time it was created. The good news is that unregistered unique identifiers are nearly all low-level iden- tifiers, generated and used by the inner workings of a digital system for tracking the data it needs. A user is unaware of them. Two examples are the UUID used within desktop PCs or the UMID used within media systems for their own inner workings. Most people quite happily use their home or office PCs without wor- rying about UUIDs, and the same is true for UMIDs when capturing or editing pictures or sound. It is important to appreciate that these sorts of identifiers are necessary for internal technical processing within a system and are not there to identify the content or intellectual property contained within it--using them for that will confuse systems and will cause system failure.