My first CD player in the '90s, a big black Philips box, had an 'index' button on the front. It allows you to jump to a position within a track, rather like having tracks within tracks, and is aimed at long classical works.

Please explain what INDEX is on a CD

I remember thinking, when I started copying vinyl LPs to CD and mp3 that it might be easier to record eack side as one track, and then add indexes at the start of each song using the timings on the sleeve. I'm glad I didn't because nearly all my subsequent players didn't support it - though my current 'hifi' player does.

Back then I didn't have any classical CDs. How I have quite a few and I've never seen one that used indexes - though I'm not sure how I'd know.

Has anybody ever had a CD that used this feature?

'a-b repeat' OTOH I used a lot when learning sings. Even my portable players could do that.