A documentation page about the user interface will be available soon. I agree that the terms should be clarified.
FrankBusse I found nothing identified as "properties pane"
The properties pane is the right part of the user interface, where you edit the properties of the selected frame and layer.
The clip button is located at the beginning of the "Layer" section (see image below).
FrankBusse briefly explain what should happen, after pressing the same
In this particular case, the effect of clipping will be visible when you play the presentation in a web browser, if the aspect ratio of the browser window is not the same as the aspect ratio used in Sozi.
We want the browser to discard the elements that are outside the rectangle that delimits the current frame.
The "Clip" button located below the timeline allows you to edit the clipping rectangle (actually, I think that I will rename this button to clarify its role).