US Navy rack mattress sizes are associated with the type of berthing personnel are assigned onboard the ship and are commonly based on their rank/paygrade. Junior enlisted personnel that are in paygrades E-6 and below are "Crew" and will share the same berthing areas. Senior enlisted personnel that are in paygrades E-7 to E-9 are "CPO" or Chief Petty Officers. Officers in paygrades O-1 and above assigned to staterooms are "Officer". Junior Officers, O-1 to O-3, on larger Naval Ships can be assigned to staterooms that accommodate 3-4 personnel. These 3-4 person staterooms have "CPO" size rack mattresses. Coast Guard Cutter berthing and other Coast Guard vessels are the "CPO" size rack mattresses. We always recommend measuring the rack and referring to our size guide to be certain of the specific size you might need.