Sadly it varies between manufacturer with regard to what it means. To my mind, what it should mean is that you can just plug in a decoder and use the loco with no problems, no soldering, no rewiring, no replacing bulbs or adding resistors. That tends to be the case reliably with Kato and Atlas and any Athearn fitted with their DCC quickplug for instance. Proto 2000 is the worst culprit, they put 'DCC ready' on their boxes for years when in actual fact you have to alter the wiring or the bulbs to prevent blown bulbs.
