747focal: The compartments on aircraft are categorised as A through D.
A class A compartment is one that the crew will access as part of their normal duties ( toilets for example ). Hand held extinguishers are easy to bring to bear.
A class B compartment is one to which the crew have access, but wouldn't normally go there without good reason ( EG lower 41 on a jumbo ). Again, hand held extinguishers can easily be brought to bear.
A class C compartment is one to which the crew don't have access in flight ( an exception to this is the fwd cargo on a 747 which is classified as a class C compartment, but you can get to it through the lower 41 if you must ) and has both fire suppression and detection; there are sub classifications of class C dependent upon the certified duration of suppression.
A class D compartment has no in flight access, and either detection but no suppression or neither detection nor suppression; fire protection being provided by the flame proof linings and the ( theoretically ) near air tight nature of the compartment.
There's a bit more to it than this, but hope this helps.