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

    The SET SOURCE command directs PCA to a specified list of
    directories for source files when source text must be displayed.
    By default, PCA expects a source file to be in the same location
    it was in when it was compiled; that is, in the same directory
    and in the same exact file. If that file has been moved to
    another location, you must use the SET SOURCE command to specify
    that location. Note that you must also establish a directory
    search list with the SET SOURCE command if you wish to view
    source files in a source window when in screen mode.