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

    Appends the performance or coverage data from the current
    collection run to an existing performance data file. If the file
    specified by file-spec exists, the Collector opens that file
    and appends the performance or coverage data from the current
    collection run to it. If the file specified by file-spec does
    not exist, the Collector creates a new data file by that name and
    writes the performance or coverage data to that file. If you do
    not specify the /APPEND qualifier, the Collector always creates
    a new file. You may append data to an existing data file only by
    using the same image that originally created the file.