For any type of file (sequential, line sequential, indexed, or relative) in your z/OS or UNIX applications, you can define the external name with either a ddname or an environment-variable name. The external name is the name in the assignment-name of the ASSIGN clause.
If the file is in the HFS, you can use either a DD definition or an environment variable to define the file by specifying its path name with the PATH keyword.
The environment-variable name must be uppercase. The allowable attributes for its value depend on the organization of the file being defined.
Because you can define the external name in either of two ways, the COBOL run time goes through the following steps to find the definition of the file:
The COBOL run time deallocates all dynamic allocations at run unit termination, except for implicit CBLQDA allocations.