"Hidden Tracks" are not meaningful for online trading and DDEX recommends that they should not be used. 

However, when a physical album is being digitised and made available in an online environment, it may be necessary to handle hidden tracks. There are two principle approaches that can be taken:

"Unhide" the hidden sound recording

The sound recording that is hidden on the physical album should be listed alongside the other tracks of the album in a separate SoundRecording composite. As a consequence, there will be a separate audio file for the Resource that was hidden on the physical album.

This is the preferred approach.

Keep the sound recording hidden inside the preceding or following sound recording

If the record company determines that a sound recording must remain hidden, it needs to amalgamate the hidden sound recording with a visible sound recording.

In that case there will be one SoundRecording composite with the title of just the visible sound recording. 

However, when identifying this sound recording, now amalgamated with the hidden sound recording, it will be necessary to allocate a different and unique ISRC from the one originally allocated to the visible sound recording. Whether or not the SoundRecording composite for the visible sound recording now amalgamated with the hidden sound recording should contain contributors for both sound recordings will be left to the record company marketing the digitised album.

There will also be a Resource file that will contain the audio of the visible sound recording plus the audio for the hidden sound recording.

This approach has the drawback that the DSP receiving such a digitised album may not be aware that there is a "hidden" recording and a "hidden" musical work contained in the file.