As discussed here, DDEX standards allow the communication of user-defined values when none of the DDEX-defined values is suitable in a specific context.
Such user-defined terms are, however, only useful to the recipient of the message if it knows, precisely, what the sender means by this term. There are, however, cases, where it is sensible to signal that a user-defined term is a specialisation of a DDEX-defined term. For example, to indicate that a specific instrument "bell lyre" is a kind of glockenspiel.
This can be communicated as follows:
This enables the recipient to note that the instrument played was not only a bell lyre but also some type of glockenspiel. As a consequence, the recipient can classify the performance by the DDEX-defined value as well as the additional, user-defined value.
The namespace indicates, as usual, the identifty of the company that has created the user-defined term (at least for the communication in this particular message).