The multi selection will only apply to initiating an interaction you've defined for that widget in the dashboard. It'll pass multiple resources to a receiving widget.
The Alerts Overview section in the Custom UI v5.8.x does have an "open alert" button, which filters the list of alerts by those of status "open". This is a user alert control state, and would apply if you're using the user controls such as "assign", "suppress", etc. Not many people use this today.
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