Library /sys$common/syshlp/dbg$help.hlb DEBUG, SET, TRACE, Qualifiers, /EVENT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /EVENT=event-name

    Causes the debugger to trace the specified event (if that event
    is defined and detected by the current event facility). If you
    specify an address expression with /EVENT, causes the debugger
    to trace whenever the specified event occurs for that address
    expression. You cannot specify an address expression with certain
    event names.

    Event facilities are available for programs that call Ada or
    SCAN routines or that use POSIX Threads services. To identify the
    current event facility and the associated event names, use the
    SHOW EVENT_FACILITY command.