ExitError (as well as unspecified exit codes) indicate a fatal error occured, but the agent should be restarted.

ExitSuccess indicates there's no need to restart the agent and everything is okay.

ExitTerminal indicates the agent has exited unsuccessfully, but should not be restarted.

ExitUpdate indicates that the agent has written an update file to the configured location and this file should be used instead when restarting the agent.