This part goes in between the ballhead and the top mounting plate so that these two parts are parallel. Then you can use the built in level to level your whole tripod and not just the top plate. After printing you can add a piece of filament in the small hole to indicate where the latch is located (makes it easier to release)
If you have any questions, feel free to ask.
and as always,
Happy printing
The author hasn't provided the model origin yet.