(Forgot where I saw this but should be some official doc):
A TEX string (and thus an expl3 string) is a series of characters which have category code 12 (“other”) with the exception of space characters which have category code 10 (“space”).
Why is it catcode 12 "other" but not 11 "letter"? Does this mean even
A-Za-z are all "other"s in a string? How does this interact with