A staple in Mainframe Interview Questions, be prepared to answer this one. The distinction between the index and the subscript: Index  The value of the index is equal to the number of places in the array that are displaced.  Index does not need a separate declaration. The index is declared using the INDEX BY notation.  Allows for a more expedient access to the data in the table.  The index is initialised using the SET command.  You may bring the index down by using the SET DOWN BY command, and you can bring it up by using the SET UP BY statement.  Subscript  The value of subscript indicates the total number of array occurrences.  A separate declaration along the lines of S9(04) COMP in the WORKING-STORAGE SECTION is necessary for subscript.  It takes longer to access the various data pieces. The subscript is initialised with the help of the MOVE command. When you want to raise the subscript, use the ADD statement, and when you want to reduce it, use the SUBTRACT statement.