The Flow deck gives the Crazyflie 2.X the ability to detect its motions in any direction. The VL53L1x ToF sensor measures the distance to the ground with high precision and the PMW3901 optical flow sensor measures movements of the ground.
This creates a flying 3D robot that can be pre-programmed to fly distances in any direction or act as an aid for beginners as it creates a very stable flying platform.
See Conpatibilitity Matrix
1 x Flow v2 Deck
Z-ranger Getting Started Guide