I don't think there's a "difference" in the actual item per se. There's no difference at all really. ECM or ECU is not a brand of computer, it's merely a generic name given to the part.

ECM = Electronic Control Module

ECU - Electronic Control Unit

Two names that usually refer to the same thing: the computer that runs a car. Now, I'm relatively new to the Volvo world so maybe these acronyms mean something else when it comes to Volvos, but in the general automotive world, they're just referring to the car's computer. Which can also be known as a PCM, Powertrain Control Module. One more to toss in the alphabet soup. ;)
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94 855T wagon