The only difference between a top level class and a class defined in an interface is the different namespace. It's used very seldomly, but if it's used at all, then typically to communicate that the class is very tightly coupled to the interface.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus