An exit code is a standard termination status.
maya encountered a fatal error Signal: 11 (Unknown Signal) Result: /users/<username>/maya/projects/Test/scenes/Test1.ma Fatal Error. Attempting to save in /usr/tmp/<username>.20120223.1655.ma // Maya exited with status 139
Each signal has a number, which is added to 128 to create the number used in the exit code. For example, signal 11 is used in exit code 139.
Signals can arise from within the process itself, or they can be sent to the process by some external agent.
When running Maya standalone on the Linux platform, if Maya exits with a 0 status when you try to end the Python process using kill -2 or kill -2 python-bin, you can set the environment variable MAYA_DEBUG_NO_SIGINT_HANDLING = 1 for Maya to exit with a non-zero status.