In the Appointment Options section at the bottom of the Configure Preferences window, users can select whether they would like to have appointments be implicit or explicit. Selecting 'implicit' makes it so that, when others try to schedule an appointment with you, any time slot on your calendar for which there is no event scheduled will be listed as an available appointment time. When 'explicit' is selected, others may only schedule appointments with you during times which you have designated as open by creating a new event and selecting Open as the Visibility option.