...In those cases, you would have to ask those who are commenting. I would guess they are setting the incidence relative to the stab. But it could be relative to the thrust line. It could be as simple as using the bottom of the fuselage.

It could just be to a line on the plans though, as it really is arbitrary. The value of "+1" incidence on the wing is completely useless for aerobatics if you don't know the tail incidence or thrust line. It's the relative positions of the 3 that make a difference.

BTW - I think when you are refering to "elevator" incidence - you are really talking about stabilizer incidence