When you install a new Unity version, you select which modules to install:
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What is the “Documentation” module for? I’ve never noticed it, but it requires a lot of memory
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Uhm it’s just the entire Unity documentation for your installed version for offline usage. It contains the same information as the Unity.com website such as the scripting reference. However unlike the online version the documentation that comes with your installation is the right one for that version and does work without an internet connection.