A rooflight is a glazed or otherwise light-transmitting roof opening that admits daylight. Products vary in form and operation. Selection depends on position and connection details as well as opening size.
How this term relates to your project
In an extension, roof glazing may bring daylight closer to the original room than rear-wall glazing alone. Consider its position alongside furniture and activities. Include shading, cleaning access and any ventilation role. The roof detail should coordinate structure, upstand, insulation and drainage.
A product dimension does not necessarily describe the clear opening or visible internal size, so establish which measurement is quoted. Specify the finish around the opening too. This makes the rooflight part of the complete room design instead of an isolated product selected after the roof has been drawn, potentially conflicting with beams or the intended lighting arrangement.