Hi folks.  Just got a phantom 3 advanced last week.  Went through the start up and wad looking for somewhere to enter the serial numbers to register.  Didn't find the procedure anywhere but it automatically came up with my email address and I pressed next and it went into the setup.
gas it self Registered or do I need To go Online to do this. As I've been flying it all week

It's activating that aircraft and if you finished the process then you are good to go, at least with DJI.
In the US, you are also supposed to register your aircraft with the FAA.