Exempt plates are issued to religious groups and other non-profit organizations. The current series begins with YYE.
Note that these plates lack the state flag in the center. The center is usually broken by a half-space. The YYM prefix, however, sometimes lacks both the middle space and the "EXEMPT" text at the bottom of the plate (see [YYM 951] below.) This is a variation in manufacturing that was not requested by the DMV.
Missing photos: YYG, YYU
Latest spotted: YYW
From AS 28.10.181
For the purposes of this subsection, "charitable organization" means a nonprofit association, corporation, society, or other entity organized, incorporated, or headquartered in the state for educational, cultural, scientific, or other charitable purposes, as prescribed in regulations of the department.