What are the differences between time-based and activity-based systems?

Asia Miles that are earned on or before 31 December 2019 have fallen under the time-based system and they are valid for three years from the month the Miles are credited.
The new rule, which came into effect on 1 January 2020, is based on an activity-based system. That means so long as you earn or redeem your Miles at least once every 18 months, the newly earned Miles will remain active*. (Remarks: *For all new Miles earned from 1 January 2020, the Miles expiry date is automatically extended by 18 months from your last Miles crediting or debiting date. All Miles credited on or before 31 December 2019 will continue to follow the former Miles expiry arrangement.)