It seems to me that both of the questions you present are related. I will attempt to explain that layer:
We have a recording, there is a original master and a remaster. They are not the same, although general speaking, MB consider them as such. Now, at the release level, those recordings can be reused. This differs from MB as it puts the mastering back on the recording.
Trying to keep this short… in today’s messed up era, ISRC plays a critical role as it relates to the digital aspect of songs. In my opinion, meaning one suggestion, is never should two ISRC’s be combined into the same recording. It is like when I started here and I was told no recording can have more than one ASIN, which is actually false, but this is the same premise as it relates to the origins and royalties.
The missing layer, to be more direct, is the mastering of the recording. See, the original recording is what it is. Now, you do in fact have a second layer of mastering … MFiT, Amazon, FLAC distributors, CDs, etc. That layer is missing. MFiT is a sore point with me, and I wish to avoid that debate here.
In short, MFiT only means that in the master they gave to produce “another master” is fine tuned with the intended second master as focus. See, with a release, you will master an already mastered recording to the release, please not that I use the term master/mastering in a certain way. On one side, you master when the different mics are combined into a recording, this is a mastering process. Now, you then take those and master them to a release. In older days, that as all the same. In modern days, MFiT is proof it now differs. I can recall proof of CDs being sold as nothing but MP3s pressed onto CD, I did not say burned, but passed to CD. We have the same today with “fake FLACs”.
Digital releases NEED specificity. I personally have MP3 releases and a “duplicate” M4A release. Are they the same? NO. They serve different purposes and are like having a release on CD and cassette. The same can be said on FLAC files of 16 bit and 24 bit, but again, not going there. I mean just to raise the point, not to debate it.
I feel as I am rambling … A recording that is listed as a recording on more than one release should sound identical on all releases it is used on, have the same meta attributes, etc. If it is unknown, it should be a unique recording, assuming we still have no intermediate layer.