/sys$common/syshlp/helplib.hlb Lexicals, F$EXTRACT, Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 start

    Specifies the offset of the starting character of the string
    you want to extract. Specify the start argument as an integer
    expression that is greater than or equal to zero.

    The offset is the relative position of a character or a substring
    with respect to the beginning of the string. Offset positions
    begin with zero. The string always begins with the leftmost
    character.

    If you specify an offset that is greater than or equal to the
    length of the string, F$EXTRACT returns a null string ("").

 length

    Specifies the number of characters you want to extract; must be
    less than or equal to the size of the string. Specify the length
    as an integer expression that is greater than or equal to zero.

    If you specify a length that exceeds the number of characters
    from the offset to the end of the string, the F$EXTRACT function
    returns the characters from the offset through the end of the
    string.

 string

    Specifies the character string to be edited. Specify the string
    as a character string expression.