often, I’ve noticed a darker/lighter band on the inside on recordable CDs, faintly seen below. not 100% sure if this always applies, or if this is exclusive to CD-R or CD-RWs…
(edit, below is a CD-RW)
I think an issue is that we’ve grouped mediums and formats together. for example, you could burn a Data CD-R quite easily, a Vinyl could have mono or stereo sound, etc. and that’s not even mentioning DVD±Rs.