Let me first say that there is no directive that will provide 100% compatibility with RM/COBOL in Visual COBOL.
There are a couple of different directives that control a certain level of RM compatible depending on the features that you are using.
If you wish to turn on certain RM syntax so that it is recognized by the compiler, such as ACCEPT/DISPLAY LINE/POSITION, etc you can do this with the RM directive.
If you wish for the behavior to emulate RM at run-time and for the compiler to adhere to strict RM standards then you can turn on the DIALECT"RM" directive.
If your migration plan is to start adding additional Visual COBOL language features to your application then you should avoid using the DIALECT"RM" directive.
Thanks.