"Block-level" tags are elements like paragraph, lists, etc. Not all these elements can directly contain text. Sequential block-level elements render one "after" another. After, in this case, can differ depending on the language. In most Western languages, "after" means below: text paragraphs are printed down the page. Other languages' writing systems can have different directionality; for example, in Japanese, paragraphs are often printed in downward columns, with the columns running from right to left, so "after" in that case would be to the left. DocBook semantics are entirely neutral to these kinds of language-based concepts.