What does ‘stack side’ mean?

what do the options Stack right, left or split stack mean?

Stack side is quite simply the side of the window the vanes will be when drawn. They are written as if facing the window from the inside of the room.

There are 3 main stack sides.

Left: When drawn the vanes (stack) will group on the left hand side of the window.

Right: When drawn the vanes (stack) will group on the right hand side of the window.

Split: The vanes (stack) will divide evenly and half will stack on the left side and half on the right.