Flow
Parameters
chapter
the spine item, zero-based.
charOffset
how far into the chapter's text the position is, in characters of reading order. Re-flowing moves the words to another page, but not to another offset. Hyphenation can shift it by a few characters; this is a reading position, not a pointer.
fragment
an element id (<h2 id="part-two">), when the position came from a link or a table-of-contents entry. It wins over charOffset when it resolves.