The phrase "seatback angle" refers to the position or tilt of the backrest of a seat. It indicates how upright or reclined the seat is. Full definition
Inside, drivers and passengers get more headroom, legroom and shoulder room, according to Audi, and split - folding second - row seats with seatback angle adjustments. (autoweek.com)
The restraints are measured with the angle of the torso at about 25 degrees, a typical seatback angle. (iihs.org)