The DS-KD-ACF2/PLASTIC is the two-module flush mounting bracket for Hikvision's DS-KD8003 modular door station range.
Flush mounting recesses the door station into the wall so the panel sits close to flat against the brickwork, which is the neater finish and the usual choice on new build and on entrances where a protruding unit would be exposed to knocks. The bracket doubles as the back box: it is set into the wall during first fix, and the modules drop into it afterwards.
Two-module size suits the most common build — a DS-KD8003 camera module plus one other module, typically the DS-KD-M card reader or the DS-KD-KP keypad. If your build does not fill both slots, fit a DS-KD-BK blanking module to close the aperture. For surface mounting instead, use the DS-KD-ACW range.