The "backrest angle" refers to the position or tilt of the back support in a chair or seat. It determines the angle at which you lean back and rest your back while sitting. Full definition
You can push the front passenger seat forward to boost the legroom (assuming nobody's in it, obviously) and the seat adjusts electronically for backrest angle and the base's location. (motoring.com.au)
That said, the well cushioned rear bench has a nice backrest angle with lots of legroom and adequate thigh support on tap. (carwale.com)
The front leg has 2 adjustable positions which will double the number of backrest angles to 12. (howtobuildmuscless.com)