Arrival time is calculated when the park brake is set and the engines are turned off. The message is only sent to BA when an aircraft door is opened (passenger or cargo). So in effect, the arrival time can be buffering and the flight is still live until a door is opened. Then the park brake set time is sent to the BA systems and the flight ‘closes’.
We (pilots) are supposed to send an additional delay message if there is a wait for stand guidance, steps, buses etc. There is usually a 2 min grace period from when the aircraft’s engines are turned off.
Unfortunately it’s a manual process and I think the data is mainly used to refine resource allocations, investigative work and for de-briefing ops teams rather than forming the basis of any official ‘disembarking’ time but I stand to be corrected.
OOOI (Out, Off, On, In) times I think are standardised by IATA as universal measurements for the aircraft journey.