A.
In the IATA Airport Handling Manual (AHM), there is a definition of Cargo Hold and Cargo Compartment.　Refer to Load Control Section AHM 501 “Terms and Conditions” where it says:

Hold : A space confined by ceiling, floor, walls, and bulkhead, used for carrying load;
Compartment : A space designated within a hold;
Bay (Position) : A subdivision of a containerized/palletized compartment, i.e., a ULD position.

A standard wide-body passenger aircraft will have 2 Holds on the main deck, i.e., the cockpit and the cabin. The cabin will have a First Class Compartment, multiple Business Class Compartments and multiple Economy Class Compartments. In the lower deck, in front of the main wing, there is the Forward Hold and behind the main wing there is the AFT Hold. The Forward Hold is divided into two compartments, FWD 1 and FWD 2. The AFT Hold is divided into 3 compartments, the AFT 3, AFT 4 and Bulk Compartments.

In the case of freighters, the main deck will house a cockpit and a large cargo hold occupying the total length of the fuselage. The lower deck is identical to a passenger aircraft.

For information, an inaccessible cargo hold during flight are located in the lower deck.

Furthermore, some airlines are mistakenly calling a Hold a Compartment. Please therefore take note.