I think that I've figured this out now. Can someone please confirm this with their Leaf/Clarion?
I wrote a USB with many music files in random order. The Clarion head played them in alphabetic order, based on the first characters of the file name.
I added some more songs to the USB and then put it back into the Leaf. The Clarion sorted them correctly. It is NOT playing in the order recorded.
Then I took those same songs and renamed them by adding random numbers in front of the names. So instead of "My Baby.mp3", the name was "01 My Baby.mp3". Sure enough, it sorted the songs by the new number that I gave each song, not the alphabetic name of the song.
Then I took those same songs and renamed them again, this time by adding the words "USB Drive" in front of the name, so "01 My Baby.mp3" became "USB Drive 01 My Baby.mp3". This time, the Clarion played the USB in random order!!!!!
So it appears that the manual is wrong. It doesn't play based on order recorded. It plays based on a sort of the first few characters of the file name, but not the whole name.
From a geek's perspective, that makes sense. It isn't worth their effort to sort based on the whole file name. Most users wouldn't care (just Rat and me). Also, sorting on the whole file name would take a little bit more memory in their tiny computer and a little bit more time when reading the USB. So they just sort on the first few characters. Note: Computer programmers never have enough memory.
And it explains why my audio book game me trouble, because the tracks on the book were files named "title 1-01", "title 1-02", etc. where title is the book title.
I assume that Clarion updates their software for each model year and each trim line (S, SV, SL) has a different software, too. So this may not apply to all Leaf/Clarion radios.
Again, if anyone could confirm this with their Leaf/Clarion, I'd appreciate it. Thanks!