"Hidden Tracks" aren't meaningful for online trading and DDEX recommends to not use them. 

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 will 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 that formerly-hidden Resource.

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

ound recording

 with a "normal" (i.e. visible) sound recording.

In that case there will be one SoundRecording composite with the title of just the normal track. 

However, when identifying  this amalgamated sound recording, it will be necessary to allocate a separate ISRC from the one used for just the normal sound recording. And whether the SoundRecording composite for the amalgamated 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 one normal 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 involved.