People misuse the term all the time, and even some users manuals are confusing because what they call "off" really means that it is at their default shutter speed, not that it is truly off. Or that to select "off" you have to select a specific shutter speed instead that means it is actually turning off the shutter (for example, selecting "1/24th" at 24P actually means turning off the shutter.)
"Off" should mean that there is no electronic shutter being used, so that if your CCD is capturing at 24P, then the exposure time is 1/24th of a second, and if you are capturing at 60i, then the exposure time is 1/60th -- in other words, the capture rate, 24 times a second or 60 times a second, is the maximum exposure time possible (i.e. you can't expose longer than 1/60th of a second if you are taking a picture 60 times per second.)
Any shutter engaged from there would shorten this maximum time, so 24P with a 1/48th shutter speed means that a shutter is being used to shorten the time from 1/24th to 1/48th, to reduce blur and match the look of a 180 degree film camera shutter angle.
If your shutter speed is longer than the capture rate -- i.e. 1/30th of a second for a camera set at 60i -- then the camera actually cannot be capturing 60 fields per second anymore because that would be breaking the laws of physics. So the camera is actually dropping the capture rate to allow the longer exposure times and then rewriting fields to add back up to the normal rate. This can mean a loss of vertical resolution in some cameras.
