I don't understand why you are confused.
I understand now, and I'm not confused. However before, when I was learning about this topic, I encountered a problem, and it's worth trying to make it easier. Even now that I 'understand', the issue continues.
It's about terminology and language, and not about the functioning of mm3.
There are distinct choices. You'd like to easily remember them, in your mind, and be able to quickly explain to yourself, or to someone else.
"list default". I would just think "Ok, clearly that's the default. Look at the name there. List default. The default."
"default processing'. I would just think "Ok, clearly that's the default. Look at the name there. Default processing. The default."
Yes, we know they're different. What is the mnemonic device to remember each one. Couldn't they be named differently?
99% of the time, you can remember "a config file", "the executable", "the bin directory", you can remember a thing based on what it's called, and those don't conflict with each other.
But if you name both of your sons "Michael" even though you understand they are separate people, it causes headaches.
If you choose a naming scheme such as "the red automobile" versus "the red car", and then ask the person "don't you understand, those are two different things for us. Why are you confused? They are two separate things. You know that. Why are you confused." Answer: the problem is the choice of naming.
So ask the question is "default processing" the only compelling name for that functionality? The word "default" is already used for "list default". Why use it again? Why not "Standard Processing -- run additional checks and accept the message." Or "Regular Processing". "Typical Processing". Anything besides "default". Invent a new word: quark-spark processing. (well, don't use 'fallback' because it's just a synonym.)
well, just trying to answer "why you are confused". maybe I am being too repetitive already.