The order of precedence is simple. There are two clases of C-T-E's,
1) No transformation (none)
7 bit, 8 bit, and Binary
2) and something to 7 bit (some).
Q-P and Base 64
There is no Binary to 8 bit transformations.
Given this grouping, the precidence can be stated as
none can be done to none
none can be done to some
some can never be done to none
and some can never be done to some.
This is an interesting abstraction for what what MIME states as a
Message and Multipart cannot be encoded from native form to 7
Message and Multipart (recursive parts) can be labeled as 8 bit or
Binary, but if you choose to encode one of these recursive parts to 7
bits, you must undo the recursion and encode the terminal body parts
as needed. Once encoded, the Message and Multipart can be labeled
properly as 7 bit.