You can turn tracing on and off from within a COBOL program either by setting the appropriate flags in the FCD or by using library routines. You might want to do this to focus on file handling problems in a subset of an application's operations. When you set FCD flags you do it for a particular file. When you use the library routines, the trace settings affect all the file operations executed by the program from that point on.
When you do dynamic tracing, you use two flags together. This is because file operations such as READs and WRITEs can be traced only if the OPEN operation was traced. If you execute a program with tracing turned off, open a file, then switch dynamic tracing on, the tracing mechanism won't be able to trace any subsequent operations. The best way to proceed is to switch dynamic tracing on for OPENs and CLOSEs at the beginning of the program, then switch tracing for all operations on and off as required. However, the tracing of OPEN and CLOSE operations on its own can also be useful.