If you enable Zoom Client App option then user will have to install Zoom Client App in their machine and if you disable Zoom Client App option then user does not have to install anything, Classes and Meeting will start on Zoom Web SDK.
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Zoom Web SDK is not as mature as Zoom Client App so may be some features will not work properly or missing in Zoom Web SDK so it is highly recommended to use Zoom Client App to avoid Zoom Web SDK issues and use full featured Zoom services.