R5 Mark II - AE for Priority Subjects During AF

With R5 Mk2 Canon introduced a new setting:

AE for Priority Subjects During AF

also known as

Detect Priority AE while AF

If this is ON only fir EVALUATIVE the metering is based on the AF point or AF area where the subject was detected.

How does this differ from what we all actually know: in EVALUATIVE metering is based on whole screen but weighted on the AF point?

If this is OFF, metering is based on the entire screen.

I don't get it...