When field installing AC and DC breakers on the same Din Rail, you should use the supplied UL listed insulator/barrier to separate the two circuits. All wiring insulation must also be rated for the highest voltage in the box. No barrier is required if different din rails are used for AC and DC.
if you are talking about that short black thin piece of cardboard (for the lack of better words), i was wondering what that was for as i hadn't run into any mention anywhere as of yet. yup, i'm guilty of not reading all in depth like many of us do. ::)