This is the End Stop Pack for the Ultimaker 2, Ultimaker 2 Extended, Ultimaker 2+ and Ultimaker 2+ Extended only.
It contains:
1X X-Axis end stop
1X Y-Axis end stop
1X Z-Axis end stop
The end stops are attached at the start of each axis and allow the printer to automatically calibrate itself every time it initiates a print or axis movement.