A data description entry specifies the characteristics of a data item.

  Format 3 -

   88 condition-name { VALUE IS   } { { EXTERNAL external-name }
                     { VALUES ARE } { { REFERENCE data-name    }
                                    { { low-val                }

                       [ { THRU    } { EXTERNAL external-name } ] }
                       [ { THROUGH } { REFERENCE data-name    } ] } ... .
                       [             { high-val               } ] }