A Sheet window is a Sheet that was opened from the project tree by clicking its name, presented as a mind map next to the tree, with the Notes as 'yellows' in it and making it possible to link the Notes.
A Sheet window and a Sheet panel overlap, so that you may need to bring the one or the other to the front. Apart from using the 'Show/Hide properties button', you can click a part of the window you need that is still visible. For the Sheet panel, that includes the project tree.
Some properties of the Sheet window can be set on the Sheet panel, such as its color, width, height and grid.