A skylight is a fixed window that doesn't open and is set into the roofline. In contrast, a flat roof
window opens for venting and can provide the flow of fresh air into the room.
It's important to mention that windows must meet British Standards, more specifically BS EN
14351-1:2010. This means that a fixed flat rooflight must be installed in the same orientation and plane
as the surrounding roof, with a minimum pitch of 15 degrees, and weathered into the roof using a skirt
or flashing.
Note that the differences between skylights and flat roof
windows may vary from one manufacturer to another, so it would be best to check a particular brand's
specifications to get more information on what features you can expect from a particular model.