On the Scheduled Visits tab, only the provider on whose schedule the patient is showing can click the "Mark as Seen" button. This button should be available to more users such as front desk staff that check the patient out to indicate that the patient is no longer at the practice. Many practices have a "provider" setup that's not an actual person (such as a lab/walk-in schedule). Since no one logs in as these "providers" then someone else needs to have access to be able to "Mark as Seen" these patients.