Num.msg is not used

I have just completed ros Beginner Level Tutorials. At Level 3 a package is created by the name "beginner_tutorials". At level 10 we add msg file called "Num.msg" to msg folder of the same package. When I made the package I made sure that Num.msg file is empty. So "int64 num" not there in the file. Later at levels 11 and 13 we add and run simple publisher and subscriber. Now Program runs smoothly. talker publish "hello world' and listener picks it up. Can anybody explain how it publish using empty Num.msg file.