The closest analog for airlines would have to be Delta. You rolled over whatever was in excess of the earned status.

As has been noted, the twist for this program is that you can choose the right to rollover before certain earned status levels.

I’m skeptical that Alaska would let you double dip miles between program years. And if I had to put money on it, I would say it’s just the right to rollover excess.

So if you chose the rollover option at 85,000 miles you keep earning in the current program year. If you end at 99,000 you can roll 10,000 over. If you end at 100,000 you are 100K and no rollover. Like Delta, this leads to some choices near the end of the year to hold off on getting to the next level.