This policy setting controls whether Outlook requires labels on S/MIME signed messages.
If you enable this policy setting, labels must be attached to all Outlook S/MIME messages before they are sent. Users can attach labels to messages in the "Message Options" dialog box by clicking "Security Settings," ensuring that the "Add digital signature to this message" check box is selected, and selecting a label under "Security Label."
If you disable all S/MIME signed messages are not required to have a label, and users cannot change this functionality.
If you do not configure this policy setting, all S/MIME signed messages are not required to have a label, but users can change this functionality through the "Message Options" dialog box.