These are list files.
The list files here simply describe what files are contained in each Debian ISO image, in order to help users find which images they might want to download. They are simply text files, compressed to save space and download time.
See the Debian CD FAQ for lots more information about Debian CDs and installation.
The images here were put together by the Debian CD team , using debian-cd and other software.
After the 9.2.0 release was made and published, a bug was found with the sorting of packages in the full DVD, BD and DLBD sets. Due to a glitch on release day, popularity contest data was not available and this caused packages to be sorted incorrectly.
This may seem like a comparatively minor issue, but it broke an important feature for some users. After the core set of packages needed for the installer, desktops, etc. are placed onto the first disc in a given set, we normally organise packages in order of decreasing popularity such that most users will typically never need to use more than the first 2 or 3 DVDs (for example). This bug broke that feature, meaning that even quite popular packages could have ended up on DVD#14 due to random sorting.
The 9.2.1 build fixes this bug. The build scripts have also been updated to ensure this bug cannot happen again in future - the build will abort if there is a problem updating popcon data.
If you have already installed using 9.2.0, you are unlikely to be affected by this bug. Live images and netinst images are totally unaffected. Apologies for any problems caused for users.