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

 expression

    Specifies the expression to be evaluated. Specify either an
    integer or a character string expression.

    If you specify an integer expression, the F$INTEGER function
    evaluates the expression and returns the result. If you specify
    a string expression, the F$INTEGER function evaluates the
    expression, converts the resulting string to an integer, and
    returns the result.

    After evaluating a string expression, the F$INTEGER function
    converts the result to an integer in the following way. If
    the resulting string contains characters that form a valid
    integer, the F$INTEGER function returns the integer value. If
    the string contains characters that do not form a valid integer,
    the F$INTEGER function returns the integer 1 if the string begins
    with T, t, Y, or y. The function returns the integer 0 if the
    string begins with any other character.