The modern EOS camera includes an orientation sensor that is typically used for several purposes, the most common and obvious is to put a marker in the images that the camera was in a vertical position and that software should display the image rotated. Since the days of EOS 30D Canon added a third option to the camera auto rotate menu, put the marker in the file but don't rotate on the camera screen, it's turned out to be the one setting I change to on all my cameras, here's why.

 If you shoot a lot of portrait images with the camera on a tripod or other support you often find you shoot some pics and want to review them. You hit review and the image is there sideways on the screen and not filling all the pixels. So you have to turn the camera round to landscape and start zooming in. With the 'put marker in the file but don't rotate on the camera' option the camera will play the image back not turned round, so keeping the camera in portrait orientation shows you an image that is filling the portrait oriented screen. But the image has the marker in it so photoshop and others turn it round when you get to the computer.

Some people don't realise that if you shoot with the 'put marker in the file but don't rotate on the camera' option set, you can change it back to rotate on computer and camera - that's what those icons mean in the menu - and when you plug your camera in to a TV for an impromtu slideshow then your images will be rotated correctly so you don't need to turn the TV on it's edge or lie on your side!