The Slide Position determines how a slide is displayed if the aspect ratio of the browser window is different from the aspect ratio of the presentation.
The slide is positioned in the viewer window so that it displays as large as possible.
The slide rectangle will either touch the upper and lower border of the viewer window, or the left and right border, or both.
This position is similar to "Fit slide in Window", but a small margin is applied.
The slide will not touch the boundaries of the viewer window.
The slide is enlarged so that it completely fills the browser window.
It's possible that some parts of the slide are not visible (cut off by the viewport of the browser).