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

    The GO command executes a program and collects performance
    or coverage data while the program runs to completion. After
    entering the GO command, you cannot enter more Collector
    commands.

    If you do not establish a performance data file with the SET
    DATAFILE command before entering the GO command, the Collector
    uses a default data file. The default data file has the same file
    name as your program's executable image file and a file type of
    .PCA. If you do not establish any data collection options before
    entering the GO command, the Collector gathers program counter
    sampling data and stack data by default.