With dynamic filename assignment, the filename is specified in the SELECT clause as a COBOL data item:
 select filename
     assign to dynamic data-item 
            	 where the parameters are:
Example
In the following example, the file input.dat is created in the current directory:
   ...
 select fd-in-name
     assign to dynamic ws-in-file.
       ...
 working-storage section. 
 01 ws-in-file     pic x(30).
       ...
     move "input.dat" to ws-in-file.
       ...
     open output fd-in-name.