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I've encountered another source of information on the topic of the Site Labels and where the road may be leading to....
BRK3101 - Securing SharePoint and OneDrive content etc..
https://www.youtube.com/watch?v=uMSulorlu7I
the video (from Ignite 2018) provides a interpretation/perspective on the term "unified" labelling.
i.e. labels defined (in the security and compliance) centre may be applied by end users across a number of different scopes/objects. i.e. at the moment they can be applied to documents but as per the compliance centre displayed in the Breakout session (which is NOT what I see when I connect to the compliance centre FYI) there will also be a means to define what happens when the same label (e.g. confidential) is applied to a Group or Site (according to the BRK session , the site settings will be aligned to the limited access/full access/no access conditional access policies in addition to settings about External Users and Public Vs Private)