It is already known that retention labels published as retention policies 'trump' retention policies created directly in the Information Governance section of the Compliance admin centre.
When retention labels are published to all SharePoint sites, they are visible ('explicit') as an option when you choose to 'Apply label to this library' in the document library or list settings. When they are published to specific SharePoint sites, they are only visible in that site, in addition to the 'global' labels.
When retention policies are created in the Info Governance section and applied to individual SharePoint sites, they normally create a Preservation Hold library that is visible (via /_layouts/15/viewlsts.aspx?view=14) only to Site Collection Admins, SP Admins and Global Admins. As these policies are invisible to end users they are sometimes described as 'implicit'. They don't include the option to review disposition before deletion.
There is nothing to stop you having a combination of explicit retention labels/policies and implicit retention policies on a single site. However, when you do this it appears to hide the Preservation Hold library BUT still deletes content in line with the implicit policy - the deleted content (deleted by 'System account') is in the Recycle Bin.
Does anyone have any suggestions how to view the (apparently) hidden preservation hold library on a site that includes explicit retention labels/policies? And no, you can't try '/PreservationHoldLibrary/Forms/AllItems.aspx'. It says 404 not found.