According to the diagram in the instruction manual, that is called a braking notch and the only mention is
If the rope is inserted incorrectly into the Click Up i.e. not following the indications marked on the device (Fig. 3.5) or the Click Up is inserted
upside down onto the harness’s belay loop so that the design with the hand and the rope is visible
(Fig. 3.6), the device will not work correctly. In both cases you must immediately stop the climb
and lower the climber as follows: whilst holding firmly onto the free end of the rope, position it
into the braking notch L. Let rope into the device by using both hands on the free end of the rope
and moving one hand then the other lower while with the other hand you firmly hold the rope
(Fig. 7.1-7.2).
In the advertisements is says,
V-shaped, tapered friction notches ensure effective belaying even if the rope is incorrectly installed in the device (i.e. the strands of the rope are inverted)
Source
So yes, it's designed to be used if you get the device upside down (which you shouldn't do) and need to lower the climber back to the ground.