Key properties define the characteristics of a key in an indexed
    file. You define keys in numerical order, starting with 0. By
    default, Key 0 always has a value of DATA_AREA 0.

    A segmented key allows you to define a key that accesses non-
    contiguous fields in a record. You can specify from 2 to 8
    segments for each segmented key in an indexed file. For example:

    o  A field in a record

    o  A field in a record that is in a record

    o  A field in a structure in a record

    You must specify a segment for each key you define. A segmented
    key can contain up to 8 segments.

    The field element or elements specified by the keyword SEGMENT
    determine the data type of the key. A key with only 1 segment can
    point to a field that has any of the following RMS data types:

       DSC$K_DTYPE_B
       DSC$K_DTYPE_BU
       DSC$K_DTYPE_W
       DSC$K_DTYPE_WU
       DSC$K_DTYPE_L
       DSC$K_DTYPE_LU
       DSC$K_DTYPE_Q
       DSC$K_DTYPE_QU
       DSC$K_DTYPE_P
       DSC$K_DTYPE_T

    See the OpenVMS documentation on RMS for more information on
    valid RMS data types.

    Each segment of a key can contain up to 255 characters. The
    sum of all characters for all key segments must not exceed 255
    characters.

    When defining a segment, you cannot use a field that is in the
    variant portion of a record.

    If you specify NULL_VALUE without specifying NULL_KEY, CDO
    automatically specifies NULL_KEY. If you specify NULL_KEY without
    specifying NULL_VALUE, the default value for NULL_VALUE is
    0. When you display a RMS database element with a NULL_VALUE
    property, the null value appears as a decimal value.

    For a description and valid values for a particular keyword, see
    the OpenVMS documentation on RMS.

    For a mapping of CDO keywords to symbolics, see RMS_Keyword_
    Mapping_Tables.