What if you have 11 units and you do horribly on the extension unit? Do they just ignore it all completely in favour of the best ten? So your ranking and scores on the extra unit doesn't matter?
Your best 2 units of English in addition to the best 8 remaining units count towards your ATAR. If you are taking an Extension unit and 11 units in total and perform horribly in the Extension unit, assuming it is the worst-performing unit compared to the rest of your units, it will not count towards your ATAR.
The confusing part is when your extension unit is good but 2 units are bad. In that case, they’ll count your extension unit but only half of the 2 unit subject towards your 10 units.