CSS Display says that, if it exists, the principal box is unique:

When an element generates one or more boxes, one of them is the principal box

However, what about an element with display: inline which is split by a block-level descendant? Then the element generates more than one inline box, so only one of them should be the principal according to the quote above, but this is not obvious.

I think all of them should be principal boxes, so the principal box is not unique and mentions of "the principal box" should say "a principal box" or "the principal boxes".