I couldn't find an answer to this on the internet: what is the 000 key?

I was wondering what that gadget in the OP was for but I think I get it ... a numpad for those who have keyboards without numpads?

I"m left handed. I hold the receipts and other papers in my right hand and use the mouse with my left hand so I wanted to have a numeric keypad on the left side near the mouse.

They also work with android devices with no keyboard and other types of computers that might have a crappy keybord but which people would use for data entry.