14 CFR §25.857

1. (a)Class A; A Class A cargo or baggage compartment is one in which—
   1. (1)The presence of a fire would be easily discovered by a crewmember while at his station; and
   2. (2)Each part of the compartment is easily accessible in flight.
2. (b)Class B. A Class B cargo or baggage compartment is one in which—
   1. (1)There is sufficient access in flight to enable a crewmember, standing at any one access point and without stepping into the compartment, to extinguish a fire occurring in any part of the compartment using a hand fire extinguisher;
   2. (2)When the access provisions are being used, no hazardous quantity of smoke, flames, or extinguishing agent, will enter any compartment occupied by the crew or passengers;
   3. (3)There is a separate approved smoke detector or fire detector system to give warning at the pilot or flight engineer station.
3. (c)Class C. A Class C cargo or baggage compartment is one not meeting the requirements for either a Class A or B compartment but in which—
   1. (1)There is a separate approved smoke detector or fire detector system to give warning at the pilot or flight engineer station;
   2. (2)There is an approved built-in fire extinguishing or suppression system controllable from the cockpit.
   3. (3)There are means to exclude hazardous quantities of smoke, flames, or extinguishing agent, from any compartment occupied by the crew or passengers;
   4. (4)There are means to control ventilation and drafts within the compartment so that the extinguishing agent used can control any fire that may start within the compartment.
4. (d)[Reserved]
5. (e)Class E. A Class E cargo compartment is one on airplanes used only for the carriage of cargo and in which—
   1. (1)[Reserved]
   2. (2)There is a separate approved smoke or fire detector system to give warning at the pilot or flight engineer station;
   3. (3)There are means to shut off the ventilating airflow to, or within, the compartment, and the controls for these means are accessible to the flight crew in the crew compartment;
   4. (4)There are means to exclude hazardous quantities of smoke, flames, or noxious gases, from the flight crew compartment; and
   5. (5)The required crew emergency exits are accessible under any cargo loading condition.
6. (f)Class F. A Class F cargo or baggage compartment must be located on the main deck and is one in which—
   1. (1)There is a separate approved smoke detector or fire detector system to give warning at the pilot or flight engineer station;
   2. (2)There are means to extinguish or control a fire without requiring a crewmember to enter the compartment; and
   3. (3)There are means to exclude hazardous quantities of smoke, flames, or extinguishing agent from any compartment occupied by the crew or passengers.