An observing mode is a set of CCD parameters: the electron-multiplying gain, the video gain and the frame time. Each mode combines them differently for bright, medium and faint sources. The camera always takes short frames, so a "total exposure time" of minutes or hours is built from many consecutive frames of that mode. Saturation is judged frame by frame: you can make the total exposure time arbitrarily long without saturating, because the extra time goes into more frames while each frame stays short.