In each show, you can specify multiple layouts for different monitor aspect ratios and orientations. Each Player will pick the layout version that better fits the specific monitor connected to the Player. This way you can have one show that can be managed and assigned into monitors with different sizes and orientations.
The standard aspect ratio is 16:9 (used by "Ultra HD", "4K", "Full HD" and "HD Ready" screens), which is a horizontal layout. You can add the 9:16 layout (the same monitor sizes, but rotated). You can also add other aspect ratios, to fit other monitor sizes, like the older standard of 4:3 monitors.