Trace statements are macros which are only active in DEBUG mode (see Section 12). If you are writing a new protocol, you should insert trace statements. Even though there will be no bugs left after you release your protocol, it may help others in debugging their protocols. Don't delete these very helpful debugging statements when you are done.
The trace levels listed in Section 8.1 are in increasing order of severity. When an x-kernel runs with tracing enabled, trace statements associated with a trace variable will print if their trace level is at least as severe as the value of the trace variable. For example, if the TCP trace variable is set to TR_GROSS_EVENTS, this will cause TCP trace statements with trace levels of TR_GROSS_EVENTS, TR_ERRORS and TR_ALWAYS to be displayed. To display all TCP trace statements, you would set the TCP trace variable to have the value TR_FULL_TRACE.