Relationship between IoTag and IoObject

I've been working for a while with Io as an embedded interpreter in C++ programs with my LikeMagic binding library, but there are still some things I don't understand.  (As Joshua Cearley said, the documentation for embedding Io leaves something to be desired; unfortunately those of us with the most to gain from the documentation existing don't have the knowledge of what to write for it!)

One thing in particular I don't understand is what is the architectural relationship between IoTag and IoObject in the implementation of Io?  And the relation of IoTag to CFunction?

I can see that:
IoObject is a typedef for CollectorMarker
CollectorMarker has a pointer to IoObjectData
IoObjectData has a pointer to IoTag

What was the reason to have some fields in IoTag versus just putting them all in IoObjectData?  Are there many IoObjects to one tag, or is it one-to-one?  Is IoTag kind of like a "type"?

I see functions for adding CFunctions with or without IoTags.  How is a CFunction without an IoTag different from one with?  Is a CFunction also an IoObject, or is a CFunction with an IoTag unrelated to an IoObject with an IoTag?  Can I create CFunctions with IoTags in a CFunction factory and use setSlot to attach them where they go?

I'm particularly interested because for my C++ binding library, it would be useful to be able to stuff an extra void* object into my CFunctions, distinct from the the target function pointer itself.  I'm hoping IoTag might be one way to do that.

The assumption with CFunctions seems to have been that you would have a unique CFunction for every unique thing you want to do, but for my C++ bindings library I find it more convenient to have a single function in C that is my only entry point from Io, and from there branch out to whatever C++ object & method should be invoked.  Currently I do this by examing the name of the message I receive to figure out what method should actually be called, but this string lookup is not so efficient.  What I'd much rather do is be able to embed a pointer to my C++ CallTarget object in a void* inside the CFunction; then in my entry point I can just cast the void* to CallTarget and call CallTarget::eval(); no lookup needed.

I think what I want is a "void* user_data;" item right under "void* state;".

While I'm looking at IoTag:  what is the difference between performFunc and activateFunc?  What is the difference between performing and activating in Io?