Item 

Description 

Single Use 

An instance of the COM server object is created for each client.  

Multiple Use 

All clients operate on a single instance of the COM server object.  

APARTMENTTHREADED 

An object is referenced only by the thread in which it was constructed.
Use this option for projects that contain only single-threaded and apartment-threaded objects.  

MULTITHREADED 

Objects can be referenced by any thread.
Use this option for projects that contain free-threaded, both-threaded, or neutral-threaded objects.  

Single 

All COM server objects are implemented using a single thread.  

Apartment 

The project may be multi-threaded, but each instance of the COM server object must has its own dedicated thread for OLE calls.  

Free 

The project may be multithreaded and each instance of the COM server object can receive simultaneous requests from multiple threads.
Your code must provide thread concurrency support.  

Both 

Same as Free except that outgoing calls such as callbacks are guaranteed to execute in the same thread.  

Neutral 

Multiple clients can call the object on different threads at the same time, but COM ensures that no two calls conflict.  

Trace Query Interface 

Sends a message to the event log whenever a client makes a QueryInterface call. The event log also shows the status of the call.  

Check Ref Counts 

message is sent to the event log whenever the reference count of a COM server object is increased or decreased. When Check Ref Counts is enabled, an assertion occurs if the project attempts to release the object from memory with a nonzero reference count.  

General Tracing 

Sends a messages to the event log when an ATL function is called.