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Windows Server 2019 is the first version of the system OS rolled out under Microsoft’s new long-term servicing channel (LTSC).
Microsoft recommends it for organizations running traditional infrastructure and applications on Windows Server. LTSC follows Microsoft’s traditional pattern of server software upgrades that consist of major releases every two or three years.
Windows Server version 1903 is a semi-annual channel feature update to Windows Server.
Note:
Semi-Annual Channel releases of Server are focused on container and applications. They don't support long-term release features like Storage Spaces Direct, software-defined networking and shielded virtual machines. Users who need those capabilities should go with a Long-Term Servicing Channel release like Windows Server 2019.
Best Regards,
Candy