There are stickers to buy with the Tux penguin, to put on that key, if that is important to you.

In Linux, the usage of this key is determined by which Desktop Environment / Window Manager you are using. for example, in the awesome window manager, it is (by default) heavily used for things like opening the applications menu, changing window layouts, window sizes, move windows, etc.
In the man page for awesome, (awesome(1)) it is referred to as 'Mod4'.