If the control area governed by a sequence-set record has free control intervals, the sequence-set record has entries pointing to those free control intervals. Each entry is 1, 2, or 3 bytes long (indicated by IXHPTLS in the header: the same length as the pointers in the index entries).

The entries come immediately after the header. They are used from right to left. The rightmost entry is immediately before the unused space (whose displacement is given in IXHFSO in the header). When a free control interval gets used, its free entry is converted to zero, the space becomes part of the unused space, and a new index entry is created in the position determined by ascending key sequence.

Thus, the free control interval entry portion contracts to the left, and the index entry portion expands to the left. When all the free control intervals in a control area have been used, the sequence-set record governing the control area no longer has free control interval entries, and the number of index entries equals the number of control intervals in the control area. Note that if the index control interval size was specified with too small a value, it is possible for the unused space to be used up for index entries before all the free control intervals have been used, resulting in control intervals within a data control area that cannot be used.