"For Hire" plates are issued to taxi cabs. The current series begins with ZZD, though the vast majority of the ones in circulation are ZZK, with ZZJ being a close second.

Missing: ZZG

The State of Alaska switched to using Centennial plates on all for-hire vehicles during the Centennial run.
I recently spotted a new E-series plate on a cab, which means that there will probably be no more Z-series plates.