Single Only one client thread can be serviced at a time. COM serializes all incoming calls to enforce this. Your code needs no thread support.
Apartment Each object instantiated by a client is accessed by one thread at a time. You must protect against multiple threads accessing global memory, but objects can safely access their own instance data (object properties and members).
Free Each object instance may be called by multiple threads simultaneously. You must protect instance data as well as global memory.
Both This is the same as the Free-threaded model, except that all callbacks supplied by clients are guaranteed to execute in the same thread. This means you do not need protect values supplied as parameters to callback functions.
Neutral Multiple clients can call the object on different threads at the same time, but COM ensures that no two calls conflict. You must guard against thread conflicts involving global data and any instance data that is accessed by more than one method. This model should not be used with objects that have a user interface. This model is only available under COM+. Under COM, it is mapped to the Apartment model.