A roto operator is what you use to open and close a casement window. When the roto is turned one direction, the closed casement window will open. When the roto operator is turned in the other direction, the casement window closes.
Because the hinge can be on the either side of the window, opening the window in the two different directions, it is important to first know which direction you want your window to open in, and then know how to make you order so you get what you want.
By convention, all windows have their right and left sides determined by how the window looks from the OUTSIDE.
If you want the hinge ON THE LEFT WHEN VIEWED FROM THE OUTSIDE, then you would order a FCL casement window.
If you want the hinge ON THE RIGHT WHEN VIEWED FROM THE OUTSIDE, then you would order a FCR casement window.