More on CSS boxes (trying again)

From the spec

Block-level elements generate a principal block box that only contains block
boxes. The principal block box establishes the containing block for
descendant boxes and generated content and is also the box involved in any
positioning scheme. Principal block boxes participate in a block formatting
context


The first sentence

Block-level elements generate a principal block box that only contains block
boxes

that doesn't make sense to me

I understand that html like

<p>
Some text<b>bold</b>
</p>

<p> is a block level element

But since it only contains inline elements, shouldn't the child boxes of <p>
be line boxes? (line formatting context)
but the spec says that <p> should only contain block boxes since it's a
block level element??

--
Sigurd Lerstad