Criterion invented special features. Not too long ago, with the invention of laser discs, the company began to include content other than the featured feature (giggle) film. They called them 'Supplements'. They do to this day, and the practice of these special features on a disc is all but expected, but Criterion often goes above and beyond. Nowadays, they often include other films, narrative, short, and documentary along with a disc. These are what I could find on letterboxd, and generally stick to actually released material that's been included on the disc, not all material per se.
Check the notes for what disc contains which release.
I'm not done yet..and feel free to suggest missing ones, wrong ones, or add more to tmdb because there's a lot seemingly missing..
From what I could find, without further ado..