In terms of deck construction, "fascia" refers to the boards fastened to the sides of the deck and stair framing to cover up the unsightly framing parts. When fascia is installed flush with the top surface of the decking, it can hide the cut ends of the deck boards as well.
Fascia is usually available in a grain and color pattern identical to each decking color, and is usually available in 12" and sometimes 8" widths.
The 12" wide boards are meant to cover 2x10 or 2x12 framing, and the 8" wide boards can be cut to length and used as riser boards. 8" riser boards can also be installed side-by-side along the stringers to create a 16" wide face that covers the 2x12 stringers and the cut ends of risers and stair treads. On smaller decks when 2x8 framing is used, an 8" riser board is often wide enough to be used as the fascia board to cover 2x8 framing as well.
Composite and PVC fascia boards are not as thick as decking, and should not be used as the walking surface on any deck.