I was on a recent flight where shortly before boarding there were a fair number of Comfort+ seats open (probably about 35% to 40%) and only one person was listed on the upgrade list. They were shown as having a seat. There was also a good amount of passengers on the stand by list when all seats in the main cabin were assigned except for 1.

I saw people on the stand by list get assigned seats in the main cabin, which I am guessing means the person originally sitting there got moved to Comfort+. Right as boarding started all seats (according to the app) in Comfort + were assigned and according to the gate television screen most if not all stand by people were assigned seats.

If everyone who falls into one of the 7 priority levels on the upgrade chart already have an upgrade, there are seats open in Comfort +, and there are still people on the stand by list waiting seats, what criteria is used to determine who to move into Comfort + to free up their seat for a stand by?