The Advanced Clean Fleet requirements for high priority fleets has been delayed indefinitely. Therefore, carriers no long have to decide between the two implementation options. The two options that were previously in place were the Model Year and Milestone phase in. Under the Model Year option, the fleet could have delayed purchasing zero emissions vehicles (ZEVs) until the existing fleet reached the end of its useful life. The con was that under this option, you could not purchase internal combustion engine vehicles (ICE vehicles). Under the Milestone option, a fleet could have purchase ICE vehicles, as long as the percentage of ZEVs in the fleet met the benchmarks. The disadvantage was that the fleet would have had to buy ZEVs on a specific schedule.