All Kontakt libraries have been formatted for a specific Kontakt version. Instruments can be loaded into a higher version of Kontakt, but not in a lower version. The required Kontakt version differs per library, depending on when the library was released/last updated. Please note that the required Kontakt version may increase with a library update.
The required Kontakt version determines the required operating system (OS).
When you download Kontakt/Kontakt Player from the Native Instruments website, this will always be the latest version.
For the OS requirements of the latest version of Kontakt, see the Native Instruments website.