Hidden reports are reports that are not available from the reports / dashboards list, but are accessed via another mechanism (usually a calculated URL field on an entity).
These reports are hidden from the reports / dashboards list because they are more practically used within the context of a specific entity. For example, the Milestone Certificate report can only be drawn for one milestone at a time and is therefore only available from the Task View page of a milestone.
The Milestone Certificate for that specific milestone will download as a PDF:
Hidden reports are shown in a hidden category on the User Group Edit page. The same rules still apply: the user group must have access to the report in order to download/view it (even though it is not accessed from the reports / dashboards list).
How do I go about setting this up?
To request a hidden report to be created, complete the normal custom report specification and clearly indicate that the report must not be available from the reports / dashboards list. Log a support ticket, attach the specification and also ask that the calculated URL field to the report be implemented.