240P is a special scan rate like 540P that shows 60 full frames per second.
240P does not exist as a video system scan rate within the industry. The reason that it does exist and we use it is because of our old NTSC 480I displays and video game consoles such as Nintendo, Playstation, etc. The reason is 240P is progressive and progressive video is a corner stone of video game programming. The speed of the scan rate is the same making 480I and 240P directly interchangeable and the display does not know the difference. Note that 240 is half of 480. For more info check progressive.
For more information go to, Scan rate.