HDCD has three elements going on that can be encoded and decoded.
HDCD uses the dither bits to encode that data.
The first element is Peak Extend
This is a reversible soft limiter. When encoded it adds some peak limiting like compression. When decoded it reverses that limiting and recovers the original dynamic range.
The second is low level gain
This adjusts the gain for very low level signals (very quiet parts of the music). Allows for smoother and more resolving fade-outs and reverb tails. Undecoded the fade-outs will have a slight stairstep in volume changes. HDCD decoding will smooth that out back to a smooth and resolving fade-out.
The third is a transient filter
The AD converter used to make HDCD recordings had an optional second filter that could be applied when recording transient sounds (like drum hits). This feature just lets you know that the AD converter used the optional transient filter during the recording process. Nice to know. But can't be recovered during playback.
All three elements are optional. They can be used or not used during the recording and mastering process to make an HDCD.
If an HDCD file has peak extend then that can be decoded to recover the full dynamic range.
If an HDCD has low level gain adjustment then that can be decoded to recover the full fade-outs and low level signal.
If an HDCD doesn't have peak extend or low-level gain then there is nothing useful to be decoded. The undecoded playback and decoded playback should sound the same.
Some HDCD titles used peak extend or gain. Some used neither. The ones that didn't use either peak extend or gain don't need to be decoded.
MC takes the approach to identify HDCD files only if they would benefit from decoding. So only if that file uses peak extend or low-level gain. If a HDCD file doesn't use peak extend or low-level gain then MC doesn't identify it as HDCD.
So you can have HDCD discs that MC won't identify as HDCD because those discs didn't use peak extend or low-level gain. Those discs are still 100% valid HDCD, but there is no benefit in doing software decoding for them during playback.
If I recall correctly, the first versions of MC that detected HDCD would flag all HDCD files even if they didn't use peak extend or low-level gain. Then sometime later the process was changed to only identify HDCD files that have peak extend or low-level gain. If you analyzed your library with one of the older versions of MC you could have some files identified as HDCD that the current versions of MC will not identify as HDCD.
I hope that makes sense. Explaining HDCD and decoding turns into a longer explanation than expected.
The Goodwin's High End web site has a library of info about HDCD. It's a good resource if you want to dig deeper into what HDCD is, or just want to understand it more.
https://www.goodwinshighend.com/library-2/hdcd-overview-library/
