I have a difficulty understanding the logic of the Sprite Editor Border.
Since we define a border to render, does that mean that anything outside the border is not rendered?
At the previous project (Snow Boarder) we set a border and the sprite was rendered based upon what was "inside " the borders, so that we avoided the round corners.
No, it does not remove anything because ‘Fill Center’ is ticked in your screenshot. However, since the inner part is transparent, it looks empty in our game. For further information, please take a look at Unity’s manual: