This new code works only with Nokia handsets with Bluetooth (BT) because it is related to Bluetooth.
If you press *#9990# on the standby screen you will get "Bluetooth test mode" and then basically what you can do is press "Ok" on the left selection key.
I am not so sure what exactly it does, aside perhaps silently checking your phone's BT module.
I just wonder, what happens if you have a defective BT???