It's subtle but it's there and its important. In electrical circuits, currents flow in loops, from the source, through a load and back to the source and it always chooses the path of least resistance. Not all electrical loads are the same of course: the return current for a throttle position sensor isn't the same as that for, say, a fuel injector. Ideally, you want to keep these sorts of currents separated from one another to keep noise out of sensitive circuits. In electronics designs, a ribbon cable (e.g.) might have one return (ground) conductor for every signal conductor to equalize impedance and minimize crosstalk. In automotive land, each sensor might have its own ground conductor (or a family of sensors might share its own ground.) Sensors may ground to one common point and all the heavier, high-current loads may ground to another. The ultimate destination of these currents, the battery, is still the same but the paths taken ensure that these currents stay separate, reducing cross-talk and noise on things like TPSs, IATs and O2 sensors.
It's important to keep power and signal grounds separated as intended to ensure things continue to operate as intended.
I am not up on my car electronics, but in general computer devices have more than one voltage in use. Typical is plus and minus 12 Vdc and also plus and minus 5 Vdc. Others are possible too. Power would be just that, the current path to the battery. PCM would have its voltages isolated from the power and so would have their own returns for each signal.
It's subtle but it's there and its important. In electrical circuits, currents flow in loops, from the source, through a load and back to the source and it always chooses the path of least resistance. Not all electrical loads are the same of course: the return current for a throttle position sensor isn't the same as that for, say, a fuel injector. Ideally, you want to keep these sorts of currents separated from one another to keep noise out of sensitive circuits. In electronics designs, a ribbon cable (e.g.) might have one return (ground) conductor for every signal conductor to equalize impedance and minimize crosstalk. In automotive land, each sensor might have its own ground conductor (or a family of sensors might share its own ground.) Sensors may ground to one common point and all the heavier, high-current loads may ground to another. The ultimate destination of these currents, the battery, is still the same but the paths taken ensure that these currents stay separate, reducing cross-talk and noise on things like TPSs, IATs and O2 sensors.
It's important to keep power and signal grounds separated as intended to ensure things continue to operate as intended.