The version, 7.12.1, in documentation / manual is a sample value so you can upgrade directly to latest v7 via stable or long-term branch. I did it with my ac2 just yesterday without any issue.
As @infabo already wrote, the 7.12.1 used to be a hard stop when upgrading from previous versions (any v6 ... but v7 prior to 7.12 as well), it is not merely "a sample value". The reason being the overhaul of how wifi/wireless packages are done and handled. Up to 7.12.1, there was a monolith optional package wifiwave2 ... which included all the wave2 drivers as well as the wifi CAPsMAN. Legacy wireless driver pack (including legacy CAPsMAN) used to be part of base routeros package. Beginning with 7.13 the package wireless was taken out of base package (it became stand-alone optional package), wifiwave2 got split into 3 parts (CAPsMAN got integrated into base routeros package, wifi-qcom and wifi-qcom-ac became standalone packages).
The package dependencies got thus broken and it was ROS built-in updater in 7.12 and 7.12.1 which was able to correctly install the new packages over the old ones without breaking running setups. After quite a while MT released some v6 release which included ROS built-in updater with same capabilities ... this the requirement to upgrade to 7.12.1 first got dropped.
The upgrade channel on V6 still gives you 7.12.1 but if we can skip this great but is there any offical documentation or communication on this as the offical guide still say
if running RouterOS v6.x, even selecting the major release upgrade channel, called "Upgrade", you will only see v7.12.1 as the available version. You must first upgrade to that intermediate version and only then newer releases will be available in the channels. This intermediate step can be done using check for updates too, but you will simply have to repeat check for updates after the first update to the intermediate version.
Thanks so much for clarifying, I used to upgrade the ac2 in the past and had problem with packages size too so I ended up using the core routeos only. This time, I just upgrade directly to v7 without any issue, that's why I was thinking the value is a sample.
I cannot test as I'm traveling but on my V6 swicthes the upgrade channel for sure points to 7.12.1 as I checked this before posting. Will check more when I get home and wonder if I need to upgrade from v6 to v7 or not.