What metric used to pick median run

In the “Performance Results (Median Run - SpeedIndex)” table, it selects a run for the “First View” and it selects another run for the “Second View”. What metric is it using to pick the media run?

Is it time to “first btye”, “largest contentful paint”, or “document complete”, or “fully loaded”?

It uses “Speed Index” (or whatever metric is next to “Median Run” in case you are running a private instance that uses a different metric.