When set to the default of On (true, yes), A_MSSQL_ADD_IDENTITY adds an extra column to any table created by Database Connectors for MSSQL. You do not have to code this column in your COBOL FD. The extra column includes the identity property and is included on all indexes that are not unique. Otherwise, when A_MSSQL_ADD_IDENTITY is set to False (false, no), no extra column is added.
You can also add an identity column using the XFD directive IDENTITY-COLUMN. Using the XFD directive gives you greater control over where it appears.
You may only specify one identity column per table, and so after the first request has been processed, subsequent requests are ignored.
A_MSSQL_ADD_IDENTITY TRUE
On keys that allow duplicates, this variable has been found to vastly improve performance. Note that the default value is True for a file that allows duplicates and is False for a file with no duplicate keys.