The NMRA's definition of Standards, Recommended Practices and Data Sheets.
An NMRA Standard: "As charged by the NMRA CHARTER and CONSTITUTION, NMRA STANDARDS provide the primary basis upon which interchange between equipment and various North American Scale model railroads is founded. Under this requirement NMRA standards include only those factors that are considered vital to such interchange. For less critical matters see NMRA RECOMMENDED PRACTICES"
NMRA RECOMMENDED PRACTICES: "The RECOMMENDED PRACTICE category was established by the NMRA Board of Trustees in January 1957, to "a. Promote maximum interchange between and within units. b. Specify the details of major components to improve design and function. RECOMMENDED PRACTICES are only less mandatory then STANDARDS by virtue of their slightly less critical subject matter and or the fact that deviation for specific reasons is permissible."
DATA SHEETS: More specific information on such topics as Scenery, Trackwork, Operation, Electrical, Motive Poser, Rolling Stock, Structures, and other general items of interest.
In the case of DCC, the electrical information and the basic packet format are included in the standard as the primary basis for interchange between equipment. Other improvements to design and function that need to be considered if manufacturers choose to implement them are included in the RP's. These are things like the extended packet format and the locomotive socket.
The standard and related Items are available on the NMRA Web Site. You can also find out how you can participate in these group discussions through the NMRA.