Illustration of indexed tables:
Index items defined through the INDEXED phrase of the OCCURS clause are one type of indexes. There can be other type of index items that are defined like data names in the DATA DIVISION with USAGE IS INDEX clause. Note that former we explained only the DISPLAY and COMPUTATIONAL usages. Index is other kind of usage. An index name defined with INDEX usage must not have any picture clause in the entry.
For illustration, the entry
77 I USAGE IS INDEX
defines the index I.
The indexes defined with the usage INDEX are termed as index data items. They are functionally similar to the indexes defined through the INDEXED phrase with the exception that when an index name is defined with the USAGE IS INDEX phrase, the similar index name can be used for subscripts in more than one table or in more than one level of table.