LTSC is for Legacy workload and Infrastructure, SAC is for Apps
When Windows Server 2016 was released Microsoft explained that the Desktop Edition and Core edition was about to be LTSC (released every 2-3 years) and Core and NANO was to be SAC (released every 18 months). The purpose was to give customers a faster “cadence”. The use case for Desktop Edition was to run legacy software that needs the UI, core was targeted for legacy software the could run without the UI and NANO was containers and modern infrastructure, like Hyper-V, Storage Spaces Direct, Network Controllers and such.
The first change to this was that in the 1709 release, NANO was now “containers” only, and what they told was from the beginning was now incorrect, NANO was no longer a choice for modern infrastructure, that was a big change and I did have customers that shifted from “happy face” to “sad face”, but ok, we can now run Core, much bigger, but fine, core will be released both as LTCS and SAC and we can use that for both legacy (in that case the LTCS edition) and for modern infrastructure (in that case SAC).
I’m happy/sorry to say that is not true anymore. The SAC release will not support anything like infrastructure roles, so, no, you cant use it in a modern datacenter, you can use it for one thing only, containers. So developers can either use NAO or Core to build applications, but if you are into infrastructure, you can run the LTCS version, and then you can either pick Desktop Edition or Core.
There are reasons for this, some of them are understandable, some of them not, but it does not matter what I think, this is happening and you need to adjust “back” to the old days.
You will use the LTCS version of Windows Server, current version is 2016 (aka 1607) and the next will be Windows Server 2019, you will upgrade every 2-5 years. You will use either Core or Desktop Edition.
You can runt whatever you want, if you like new features, go for SAC, upgrade/reinstall/redeploy every 18 months or less, or do old style, go for LTCS
You can read the “official” statement here.