PQ codes are the parallel data tracks that define certain aspects of CD metadata â€“ most importantly track start, track end, pauses, and some metadata codes.Â  Generally you donâ€t have to deal with these directly, unless youâ€re trying to do something clever like put hidden tracks before track one or whatever.Â  CD creation software handles this stuff for you and usually hides it behind friendlier â€œtrack startâ€ and â€œtrack endâ€ interfaces.