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Guest User is the meta-user whose layout is presented as the (unmodifiable) layout of the unauthenticated user. A portal maintainer "logs in as guest" to adjust the "guest layout". Changes to the guest user's layout are reflected in the unauthenticated view on the portal after the portal is next restarted, since the guest layout is aggressively cached.
You can have (in theory anyway) have more than one Guest User and different unauthenticated layouts among which your portal switches (say based on the IP address from which the request originates – an on campus unauthenticated view and an off campus unauthenticated view – but this is an advanced usage and probably doesn't work perfectly since probably no one is actually exercising it.
How do I log in as the Guest User?
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