So there is the version of firmware you are currently running which is the version of Horizon, the switch's operating system, this is called "Official firmware" or OFW and then there is some version of patches to the Horizon OS that is called custom firmware or "CFW".
So Atmosphere CFW v0.12.0 supports Horizon/OFW 10.0.1 currently (and all previous OFW versions). It is also possible to do Atmosphere CFW 0.12.0 on OFW 9.2 and everything will also work. Running OFW without CFW is known as running the "stock" firmware, which means that Horizon is running without any custom patches applied.
Changing the versions of patches applied has nothing to do with the Horizon version besides needing to update them occasioanlly a.k.a. updating the CFW.
The deal with fuses is that, if you are not using "recovery mode" to boot the switch, they will be burnt whenever booting into Horizon from a coldboot state and each OFW requires a certain # of fuses burnt. coldboot->power button->OFW = burnt fuses. So without recovery mode, booting into OFW from coldboot will not be possible when using an older version of Horizon (if Horizon has been downgraded) because too many fuses have already been burnt. That is probably what you are hearing on forums a lot. That part is not nonsese.
However, this fuses problem is bypassed 100% by enabling recovery mode which skips the fuses check/burning process completely, so then you can do "cold boot->recovery mode->OFW" or "cold boot->recovery mode->CFW" and no fuses are ever burnt or checked. There is no reason to downgrade, but even if you ever did, all you would have to do is to enter recovery mode and then boot into OFW for everything to work normally.
So Atmosphere CFW v0.12.0 supports Horizon/OFW 10.0.1 currently (and all previous OFW versions). It is also possible to do Atmosphere CFW 0.12.0 on OFW 9.2 and everything will also work. Running OFW without CFW is known as running the "stock" firmware, which means that Horizon is running without any custom patches applied.
Changing the versions of patches applied has nothing to do with the Horizon version besides needing to update them occasioanlly a.k.a. updating the CFW.
The deal with fuses is that, if you are not using "recovery mode" to boot the switch, they will be burnt whenever booting into Horizon from a coldboot state and each OFW requires a certain # of fuses burnt. coldboot->power button->OFW = burnt fuses. So without recovery mode, booting into OFW from coldboot will not be possible when using an older version of Horizon (if Horizon has been downgraded) because too many fuses have already been burnt. That is probably what you are hearing on forums a lot. That part is not nonsese.
However, this fuses problem is bypassed 100% by enabling recovery mode which skips the fuses check/burning process completely, so then you can do "cold boot->recovery mode->OFW" or "cold boot->recovery mode->CFW" and no fuses are ever burnt or checked. There is no reason to downgrade, but even if you ever did, all you would have to do is to enter recovery mode and then boot into OFW for everything to work normally.