What is this key for? What is it called?

Question

I have searched and searched, on Google, Wiki etc. but there's no mention of this key, not even in the manual:
http://www.paulofierro.com/images/secondaryEnterKey.jpg
On my MacBook Pro, it is placed in the bottom, between the Apple-key and the left-arrow key (on the right of the space bar).
What is it's use? What is it called? And why is the symbol also on the enter-key?

Answer 1

On my MBP it doesn't have a symbol, but instead says "enter," and has the exact same functionality as the "enter/return" key.
It's there for using the numpad. If you hit "num lock," then use keys 7,8,9,u,i,o,j,k,l,m,. it turns into a numpad equivalent to what's on the right edge of desktop keyboards. Much like a desktop, while you're using the number subscripts on these keys, there's always an enter key at the bottom right edge (makes it easier for entering long sets of numbers).
Even without num lock on, this secondary enter key still functions just like the primary enter key for regular functions.