Represents flags for RDB$PARAMETERS system table.

    Bit
    Position   Description

    0          IN (read) INOUT (modify).
    1          OUT (write) INOUT (modify).
    2          Reserved for future use.
    3          BY DESCRIPTOR (default is BY REFERENCE).
    4          BY VALUE (Bit number 3 is ignored).
    5          Reserved for future use.
    6          Set if parameter mode is undefined.
               If Bits 0 and 1 are both clear, then the parameter is
               the RETURN TYPE of a function.