An indexed file contains records ordered by a record key. The record key uniquely identifies the record and determines the sequence in which it is accessed with respect to other records.
Each record contains a field that contains the record key. A record key for a record might be, for example, an employee number or an invoice number.
An indexed file can also use alternate indexes, that is, record keys that let you access the file using a different logical arrangement of the records. For example, you could access the file through employee department rather than through employee number.
The record transmission (access) modes allowed for indexed files are sequential, random, or dynamic. When indexed files are read or written sequentially, the sequence is that of the key values.
EBCDIC consideration: As with any change in the collating sequence, if your indexed file is a local EBCDIC file, the EBCDIC keys will not be recognized as such outside of your COBOL program. For example, an external sort program, unless it also has support for EBCDIC, will not sort records in the order that you might expect.
related references
Valid COBOL statements for indexed and relative files