Flush tank component with unknown function

Dear people, I don't know what to do. I just got the following kit from Siamp, but at home I found out that I couldn't do anything with one component. So back to the hardware store, where the seller also didn't know what the thing was for. In the picture below, it is the part at the top right corner, the one with the two feet. Does anyone of you know what it is and what it is used for?

Did you buy that separately or was it part of the flushing cistern? In any case, it is the stopper for the level of the flush water.

Hello,

the small caps are there to close the side connection in case the connection comes from the back.
See photo.