Slab needs to rely primarily on friction, and mostly delicate footwork, minimal handwork. Whitehorse Slab is the best example.
Once it is too steep for relying primarily on friction, I consider it "low angle face." Low angle face can still be slabby in appearance and texture, but you are mostly edging or smedging on those feature vice utilizing friction (and the bolts generally become more normally spaced, vice "slab" spacing).
Face climbing would be the stepper version of "low angle face" and would generally be more featured, either bolt protected because of the inability to adequately protect with trad gear, or featured enough for trad protection.
To me, the idea of "steep slab" seems impossible because your cant "friction" up "steep" slab ... thus I call it "low angle face"