PAL uses a screen resolution of 720 x 576 pixels and has a refresh rate of 25 frames per second.
NTSC uses a lower resolution of 720 x 480 pixels, but a higher refresh rate of 29.97 frames per second.
If the linked clips are not in either full resolution PAL or NTSC, we will display a fail symbol: 
If the linked clips are in full PAL or NTSC resolution and have been enhanced with QTGMC (same resolution, but double framerate), we display this symbol:
576i on wikipedia
480i on wikipedia