A trace file is used to search for and correct errors. It logs what SAProuter does (the higher the trace level, the more detailed is the information). From this, you can see in which function an error occurred, why a connection was not established, etc.

When you start SAProuter, you can specify a trace file:

There is always a trace file. If the option is not used, the trace file dev_rout is used. It resides in the working directory of the SAProuter.