I am not a Tesla insider (I am a retired IBM software engineer) but as I understand it, Tesla's releases are snapshots of a given week's software level. Eg: 36.8 is the eighth version of the week 36 software level. Tesla almost certainly follows a software development process called 'agile' where both new function and fixes to current function are continuously added to a development stream. That stream is then checkpointed at intervals, tested, fixed/updated as necessary (thus the eighth version of the week 36 build) and then released to users. The release is done in a staged manner to groups of users rather than to everyone at once so that if there is a problem only a subset of users will be affected. If a serious problem is found after the initial release, then you might see a 36.8.x 'fix' build for example.
If the above is correct, then when (for example) I get 40.6 I will not only get Tidal support and a tone when TACC is turned on (things that were added to the development stream after week 36), but also any fixes that were implemented between week 36 and week 40 even if Tesla doesn't explicitly talk about them in the release notes.
That takes me to the FSD release stream. I have to speculate here. Possibly it is an entirely different stream to which are added changes from the 'regular' stream (or vice-versa - the regular stream gets stuff from the FSD stream...) or possibly there really is only one stream but the FSD function is either walled-off in or deleted from the non-FSD releases and then the FSD version is given a unique version number. The latter would make more sense from an external point of view but perhaps Tesla has some good reason for keeping them completely separate.
It is in Tesla's best interests to have as few development streams as possible since each stream needs extensive testing and that is expensive. Likewise, it is in Tesla's best interests to have as few releases to support in the field as possible. Where they do maintain multiple/separate releases, they must have a good reason such as hardware-specific support, local regulations, or the staged release cycle that I mentioned above. When you see something like my experience of getting 3.106 after 4.18.1, it must have been worthwhile to Tesla to update the week 3 build with function/fixes from later builds rather than move my car to a later week's build (or stay on week 4 with those updates). I will guess that they did that because I have one of the unique chipsets which were being implemented this summer and it took until the week 32 build for support for my chipset to be part of the normal/common development stream.
I would expect that in the future there will only be one development stream and that FSD will always be present but gated by a software switch much like the acceleration boost for LRs or heated seats for standard models.