Partially checked means that application update check or extension update check is enabled but not both. I’ll add an explanation to the tooltip.
How does this message look in the about window?
If there is no update then the window would look like this:
If application update check is disabled in CMake then the Visit the <a href="https://download.slicer.org/">download site</a> to check if a new version is available. message would be displayed.