How Does SEPM Determine if a Client Is Online or Not?

I currently manage a SEPM server running on a 12.1.3 managed configuration and I am trying to figure out a few things that have popped into my head but have never been figured out…

1. How does SEPM determine the online status of a managed client?
   • Does it ping the NIC on the managed machine?
   • Does a user need to be logged in?
2. Does a user need to be logged  into the client machine in order to update the definitions or will it do it automatically, so as long as the client is in an ONLINE status?