FLStudio's plugin manager is not up to date with the latest versions of the SDK.
This was taken from the page "VST 3 Locations / Format" in the SDK documentation:
[quote]
On the Windows platform a VST 3 Plug ...
FLStudio project files with FLStudio so that if you double-click on a .flp file, FLStudio will open automatically. NOTE: If you are using more than one version of FLStudio, use this option to associate files ...