In the Air Vent Installation Instructions what is the purpose of the sub-fascia board shown in the illustration for Vented Drip Edge?

When installing Vented Drip Edge the gutter may need to be blocked out away from the fascia board using a sub-fascia board to minimize water overshooting the gutter.

In some cases the gutter may need to be blocked out away from the fascia board using a sub-fascia board to minimize water overshooting the gutter. That is why it's shown in the Installation Instructions. The board is likely a nominal 1 x 3 or 1 x 4.