For the trace to be of any use for multi-threaded or multi-process applications, you will need to use the TRACEPATHNAME keyword. Otherwise the trace will be garbled if multiple threads or processes are writing to it simultaneously. See TRACEPATHNAME for more information.
The files are created in the path specified with the name set to the process id of the application and an extension that is a sequence number for each unique thread (eg. 65397.0, 65397.1, 65397.2 etc.).
By having each thread write to its own file, no semaphores are needed to control access to the tracefile, which means tracing doesn't change the behavior of a multi-thread application.