From Halopedia, the Halo wiki
The air frame of the Pelican allows them to carry heavy objects underneath their tail booms. Common carry items include an M12 Warthog, M808 Scorpion tank, troop deployment pods or resupply canisters.
Pelicans receive serial numbers to be identified by during radio transmissions, particularly in combat situations. Each serial number consists of a letter from the Phonetic Alphabet followed by a number. Though a few Pelicans have this number inscribed on their cargo bays, most have no distinguishing mark on their exteriors.
Unidentified Pelican encounters