From Halopedia, the Halo wiki
The CAR-class frigate is a classification of frigate within the now-defunct Covenant fleet. Several frigates of this class participated in the Fall of Reach. The "R", or Reverence, in the frigate's three-letter classification code, "CAR", denotes that the frigate is capable of conducting reconnaissance missions and is capable of excavating Forerunner artifacts.
Ships of the line
The CAR-class frigate was first mentioned in Transmissions, a type of promotion for the then-new game Halo: Combat Evolved. The transmissions are directly linked to the Cortana Letters and are considered canon because of their inclusion in the Halo Encyclopedia (2009 edition).