A hammer drill often has a chuck and as a result the amount of hammer is limited so as not to damage the chuck. With SDS drills you have no chuck so they can use a lot higher hammer action and some also have a rotor stop so you can fit a chisel and mine quite easy cut through an old fence post including the steel reinforcing bars. Some have reverse very handy when drill gets stuck.
I have not seen term impact drill but I will guess that's what you are talking about.
A band jack with a huge compressor is still an impact drill the term drill does not mean it has to turn look at a seed drill it just has to penetrate something.
Does this help.
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