I was excited to find “TAB6” and “TAB4” under “clefs” at Steinberg.help, but that disappeared when I tried to use them. I created a single player with a 6-string acoustic guitar and changed the clef to TAB6 using the popover. The clef now reads “TAB” but there are still 5 lines, and it shows notes, not numbers.
Are these perhaps for “future features”? 
I wouldn’t read too much into it. I mean, Dorico might support tab-notation in the future and it is quite interesting that you can already call the glyph through the clefs popover, but Bravura has implemented all kinds of symbols that have no immediate use in Dorico because, as flagship implementation of SMuFL, it has the duty to implement the whole range.
And now I’m disappointed that you can pull TAB as clef and not Lachenmann’s bridge clef…!
Thanks LSalgueiro. My excitement was short-lived, but still I am hoping that their presence does point to a future direction. I just hope it’s not the distant future.
It won’t be the far distant future, but I’m afraid there won’t be any guitar-specific features like tablature or chord diagrams in the Dorico 2.x series of releases. It’s a high priority for the version beyond, though.
Thank you Daniel. Not the happiest of news but good to know that we can start looking forward to 3.x now! 