Difference between "free-threaded" and "thread-safe"

I've just done some research on what "free-threaded" might mean, and I also ended up with COM. Let me first cite two passages from the 1998 edition of Don Box' book Essential COM. (The book actually contains more sections about the free-threaded model, but I'll leave it at that for now.)

A thread executes in exactly one apartment at a time. Before a thread can use COM, it must first enter an apartment. […] COM defines two types of apartments: multithreaded apartments (MTAs) and singlethreaded apartments (STAs). Each process has at most one MTA; however, a process can contain multiple STAs. As their names imply, multiple threads can execute in an MTA concurrently, whereas only one thread can execute in an STA. […]

— from pages 200-201. (Emphasis added by me.)

Each CLSID in a DLL can have its own distinct ThreadingModel. […]

• ThreadingModel="Both" indicates that the class can execute in either an MTA or an STA.
• ThreadingModel="Free" indicates that the class can execute only in an MTA.
• ThreadingModel="Apartment" indicates that the class can execute only in an STA.
• The absence of a ThreadingModel value implies that the class can run only on the main STA. The main STA is defined as the first STA to be initialized in the process.

— from page 204. (Formatting and emphasis added by me.)

I take this to mean that a component (class) who is declared as free-threaded runs in an MTA, where concurrency of several threads is possible and calls to the component from different threads is explicitly allowed; ie. a free-threaded component supports a multithreaded environment. It would obviously have to be thread-safe in order to achieve this.

The opposite would be a component that is designed for an STA, ie. only allows calls from one particular thread. Such a class would not have to be thread-safe (because COM will take care that no other thread than the one who "entered" / set up the STA can use the component in the first place, that is, COM protects the component from concurrent access).

Conclusion: COM's term "free-threaded" essentially seems to have the same implications as the more general term "thread-safe".

P.S.: This answer assumes that "thread-safe" basically means something like, "can deal with concurrent accesses (possibly by different threads)."

P.P.S.: I do wonder whether "free-threaded" is the opposite of "having thread affinity".

“free-threaded” and “thread-safe” have different meaning. For example, ASP.NET can use "application state" to share data between different web sessions and users of a application (aka IIS virtual directory and all subdirectory). Microsoft Help said:

https://docs.microsoft.com/en-us/previous-versions/aspnet/ms178594(v=vs.100)

Application state is free-threaded, which means that application state data can be accessed simultaneously by many threads. Therefore, it is important to ensure that when you update application state data, you do so in a thread-safe manner by including built-in synchronization support. You can use the Lock and UnLock methods to ensure data integrity by locking the data for writing by only one source at a time.

Therefore, I understand the "application state" can be read simultaneously; but to update its contents, your source codes of ASP.NET must use lock and unlock operations around.

It can extend the understand to COM domain. “free-threaded” for MTA apartment means threads can read COM data simultaneously；but to update COM data in MTA, your source codes should use synchronization manner by yourself. In the other hand, STA is “thread-safe” by COM itself. Your program can access STA data diretly and easily.