You use directives to control the way the Compiler behaves: what output it produces, what code is compiled and how the compiled code behaves when it runs.
You can specify COBOL compiler directives within the IDE and in a number of ways such as directly on the command line or within your code using a $SET statement. You can also use the directives.mf file to specify which directives to use when compiling COBOL applications from the command line.