What do the M4700 function keys do?

I've a new Precision M4700 and am trying to figure out what all the Fn function keys do. I think I've figured out all but one of them.

Fn+F1 (moon symbol) - put it to sleep

Fn+F3 (Lock with down arrow) - I haven't a clue

Fn+F5 (Touchpad symbol) - Disable/enable the touchpad

Fn+F8 (laptop/monitor symbol) - Toggle multiple monitors

Fn+Home (SysRq) -

Fn+End (Print Screen) - Print screen

Fn+Insert (Pause)

Fn+Up or Down arrow (Sun symbols) - Make display brighter or darker

Fn+Right arrow (glowing symbol) - Toggle the lighted keyboard light

What does Fn+F3 do? It does not seem to do anything.

Brian