File Extension CLS File Format Visual Basic Class Module CLS stands for Visual Basic Class Module. This was last updated in March 2012 By Margaret Rouse Browse File Extensions Alphabetically: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # Ask a Question. Find an Answer. Ask your peers a question about this file extension. Ask a Question Interfaces and Properties how to stop execution of a code until a Jwindow is closed? How you change a pointer from base class to derived class?