MSDN Blogs: X++ in AX7: Internal keyword

internal is a new keyword in X++. It has the same semantics as in C#.

When you mark a class or method as internal, then it can only be accessed from within the model where it is defined.

internal class MyInternalClass 
{  
    internal void myInternalMethod() 
    {  
    }  
}

Notice, you can define internal methods on public classes too.