Morten Stenshorne
Block fragmentation is splits a CSS block-level box (such as a section or paragraph) into multiple fragments when it doesn't fit as a whole inside one fragment container, called a fragmentainer. A fragmentainer is not an element, but represents a column in multi-column layout, or a page in paged media.
For fragmentation to happen, the content needs to be inside a fragmentation context. A fragmentation context is most commonly established by a multi-column container (content is split into columns) or when printing (content is split into pages). A long paragraph with many lines may need to be split into multiple fragments, so that the first lines are placed in the first fragment, and the remaining lines are placed in subsequent fragments.