What is a 0-size file "Icon?" in a directory? I just noticed in my ~/Copy directory there is a file named Icon? that is 0 byte.

The file is only visible by a ls command.

I'm just curious what for this file exists, does anyone know about it?
It's just an empty file. Nothing more. 

Since the ~/Copy is not a standard directory, it is hard to say why is it there or why it was created.