Positions 63 and 64 indicate the fields that are used as control fields. A change in the contents of a control field causes all operations conditioned by that control level indicator and by all lower level indicators to be processed.
A split control field is a control field that is made up of more than one field, each having the same control level indicator. The first field specified with that control level indicator is placed in the high-order position of the split control field, and the last field specified with the same control level indicator is placed in the low-order position of the split control field.
Binary, float, integer, character varying, graphic varying, UCS-2 and unsigned fields cannot be used as control fields.