4.9.3.2 Hidden Tracks
Although currently it is not possible technically to embed an ISRC into a hidden track on a Compact Disc, it is nevertheless recommended that an ISRC be assigned.
Translation: Even though it's impossible to do, we want you to go ahead and pretend that you did it anyways.
![:roll:](http://www.cdrlabs.com/forums/images/smilies/icon_rolleyes.gif)
taken from here:
http://www.ifpi.org/isrc/isrc_handbook.html