A DTC or Diagnostic Trouble Code, is a code produced by an engines ECU, to assist in rapid fault correction. ECU's do not use standard DTC and as such each engine manufacturer may have several different sets of codes for different engines.
The DTC's are usually read down a CANBUS connection to the generator control module and displayed on the generator controllers screen. Some manufacturers may choose to hide the codes from the user and require a call out to diagnose or view the codes.
The DTC is used to identify where the engine sees a problem, this problem can then be checked and rectified. For this reason ECU's can save significant amounts of time trouble shooting engines, as the ECU can with the help of the DTC point the engineer in the direction of the fault.