I suspect that there is an error in the Turtle specification, in the section shown in the screen-shot.
(It relates to JSON-LD, which ActivityPub / ActivityStreams is built on.)
I suspect that "PN_CHARS_BASE" is an error.
Because other parts of other specifications seem to not make sense if it is.
I suspect that maybe it should have been "PN_PREFIX" instead.
PN_LOCAL is exactly what the prose in your screenshot describes. I see no error.
What other parts of other specifications don't make sense?
One example is...
I've seen multiple examples of Blank Node Identifiers (which contain a Blank Node Label after the "_:") that include numerical digits ("0", "1", "2", "3", "4", "5", "6", "7", "8", "9").
PN_CHARS_BASE does not seem to include numerical digits.
Neither does that section "2.6 RDF Blank Nodes" include numerical digits.
Which, assuming I didn't misunderstand something (which is possible), makes the examples I've seen elsewhere not make sense (since they would be invalid).
I think that's what they mean by:
> The characters _ **and digits** may appear anywhere in a blank node label.
(The first of the three liberalisations in your screenshot.)
I completely missed that. Thanks for pointing out.
@reiver ⊼ (Charles) 
Nik | Klampfradler 🎸🚲
Jan Martin Keil