This from Antec:

In evaluating a power supply, one important thing to look for is the “+3.3V and +5V combined output” rating. The total amount of power you can draw from those two output circuits combined is typically much less than the sum of the individual ratings for 3.3V and 5V listed elsewhere on the box. An example from a competitor’s 430 Watt model:

Thus, according to the label, if one fully loaded the 5V circuit in accordance with the manufacturer’s specifications, there would be essentially no power left on the 3.3V line. Note that the total power available on the 3.3 and 5 volt circuits combined is barely more than half of the 430 Watt rating of the unit as a whole. TruePower has dedicated output circuitry for each voltage line, which means that there is no combined output limit other than, of course, that of the entire power supply. Each voltage line can perform up to the specification on the label, without regard to or interaction with the loads on the other lines, until the full capacity of the power supply itself is reached.

A lot of us have probably had various problems caused by connecting too many things on one line. Antec is saying that is no problem with the TruePower line of power supplies. Not that you should connect everything to one line, but if you have lots of toys as I do such as fan controllers, 9 fans, etcc, this is a major plus. On non-TruePower power supplies connecting too much on one line could lead to assorted fun things like blue screens, reboots, lockups and even a blown power supply or other system components.

See the full specsheet on the TrueControl 550 at Antec