Answer supplied by Eva Verdugo:

The Herschel Science Archive includes in the “Basic Search” panel a field called "Access Status" that allows searching by "Public", or by "Restricted" observations. Some observations were taken in non-standard modes, or with special set-ups only used for calibration and engineering purposes and their access is therefore restricted by default to Herschel experts only. If for any reason you still think you need to gain access to any of these observations please raise a ticket at this HSC Helpdesk indicating the reason and we will assist you providing technical advice on the additional caveats you need to consider and the limitations affecting the observation for any potential use.