The NMRA has defined several Decoder Sockets and plugs to allow end user the option of installing decoders in a "DCC Ready" engine with a simple "Plug and Play" setup.
1) DCC READY
DCC ready is a term not defined by anyone. It means different things to different locomotive manufactures. At a minimum, the term that applies to DC locomotive that is:
a) wired to make installing a DCC decoder easy. The engines frame is not used to carry any electrical current. The motor is electrically isolated. Everything electrical is done with wires.
b) made with physical space available to accept a DCC decoder.
The idea behind the DCC ready locomotive is to offer a quality decorated engine that works on plain old DC for the lowest possible cost but yet allow for post sale easy upgrade to DCC operation later without making the DC user pay for it. These electrical and mechanical locomotive design where limited mostly to the first generation of locomotive that were designed in a time where the manufacture were now fully aware of the growing DCC standard and the need to somehow support it. Today, DCC Ready now means the same things as DCC Plug and Play
2) PLUG AND PLAY
"Plug and play" is a term the defines a specific subset of DCC Ready engines. The locomotive is still a DC locomotive out of the box. Plug and Play refers to the ease of installing a DCC decoder post sale inside the given engine by means of using a standardized DCC connector. It means you do not need to work with wiring inside the locomotive to get the decoder installed and work including all the lights. Remove the shell, install the appropriate Plug and Play DCC decoder for the given engine and then reinstall the shell. Your ready to go. TODAY almost all DC locomotive manufactures have embraced the Plug and Play version of DCC ready since even DC locomotives now have some simple electronics for DC use relating to constant lighting and any other light based animation. The electronics requires a PCB board and the PCB makes it very easy to a DCC socket/plug. Win-Win. Until the 21 Pin DCC connector is defined, there is no Plug and Play for sound decoders.
3) DCC PLUGS/SOCKETS
Below is a list of the most popular DECODER plugs and Sockets standards.
A) Any outputs above the two headlight outputs are optional per the NMRA standards. See your decoder specification for support.
B) In 2008, the NMRA introduced a 21 pin NMRA standard based on the 21MTC standard developed by Marklin and ESU. However the standard had some flaws that other DCC manufactures using 21 pins did not follow. In 2010 the NMRA marked the standard as follows: Under Review, 21MTC and PLuX connectors are being modified and/or removed. For more information, go here: 21 Pin Plug Issues