The OPEN, CLOSE and EOV functions support a function called module flow tracing. It is for debugging system code and can help with debugging the OPEN, CLOSE and EOV exits that are described in Tape Label Processing Installation Exits. You can request this function by coding DIAGNS=TRACE on the DD statement. You also must start generalized trace facility (GTF) to trace USR events. z/OS DFSMSdfp Diagnosis describes OPEN, CLOSE, and EOV module flow tracing. z/OS MVS Diagnosis: Tools and Service Aids describes GTF.