What are DRM-protected songs and can I play them in djay for iOS?

Some songs purchased from the iTunes Store are protected by Digital Rights Management (DRM).

djay for iPad and iPhone supports all tracks purchased on the iTunes Store with the following restrictions:

• Apple Music: Tracks added to your library via Apple Music can not be played.
• DRM protection: Older tracks purchased from the iTunes Store can have DRM protection and cannot be played.