The shortest part, and it turns from the chain to the text itself. Only two chapters: how the compilers wrote their chapter headings, and the remarks they attach to a text as opposed to a chain.
Chapter headings, tarajim al-abwab (تراجم الأبواب), are where a compiler quietly states his own view. This chapter sorts them into three kinds and shows the different routes a compiler takes to build one.
Headings in the two Sahihs - and the two could hardly be more different, since one of them wrote no headings at all.
Al-Bukhari split his Sahih into 97 books, each into chapters, each with a heading. Baqa'i sorts them into three kinds by how obvious the link to the hadiths is: plain, hidden, and 'released'. It is in these headings that al-Bukhari's own legal reasoning lives.
The plain heading, where the link is obvious and needs no thinking. Several routes: quoting the hadith's own words, or part of them, or its sense; asking a question ('is such-and-such the case?') where al-Bukhari does not want to commit; borrowing wording from a verse; or a broad statement, or a narrow one.
The hidden heading, where you have to work to see the link - and this is where the difficulty of al-Bukhari's book really lies. Typical moves: a broad heading that the hadith then narrows down, or the reverse; a heading that looks trivial but is actually a rebuttal of someone; or a link that only follows by implication.
The 'released' heading, where al-Bukhari writes only the word bab and gives no title. These usually carry hadiths belonging to the chapter just before, so the untitled chapter works like a subsection of it. Baqa'i counted them: only two in the Book of Faith, two in Tayammum, three in Prayer.
Muslim wrote NO chapter headings. He arranged his book by chapters and did so carefully, but left them untitled. The headings in printed copies are al-Nawawi's, who said the earlier attempts were poor and supplied better ones - all of them plain headings, and some rather long.
Headings in the three Sunan. The big difference from al-Bukhari: these are mostly plain headings, and the hidden reasoning-style heading is rare.
Their plain headings use the same routes al-Bukhari used, and Baqa'i lines up examples from all three side by side: the question form, the broad statement, the narrow statement, and borrowing the wording of a hadith or verse.
Their hidden headings, also matched to al-Bukhari's routes. The commonest two: a heading broader than the hadith under it, and a heading narrower than it. A third is the heading that looks pointless until you see it is answering an objection.
The three Sunan authors also leave chapters untitled sometimes, writing only bab, or 'another of the same kind'. Rarer here than in al-Bukhari - which is saying something, since they are rare in al-Bukhari too.
The mirror of the chapter with the same name in Book Two. That one collected remarks about the CHAIN; this one collects remarks about the TEXT, in three sections: hard words, abrogation, and apparent conflicts.
Gharib al-hadith (غريب الحديث), explaining rare or difficult words in the text. Abu Dawud and al-Tirmidhi do a lot of this, al-Bukhari some, al-Nasa'i occasionally. Baqa'i's closing observation: these explanations are not the compiler's own wording at all, he transmits them from earlier authorities.
Marking which ruling replaced which. Al-Bukhari, Abu Dawud and al-Tirmidhi sometimes say outright that a hadith is abrogated, or that this was the last thing the Prophet did. Other times they only imply it by the order in which they place the hadiths, without saying the word.
Two things at once. Mukhtalif al-hadith (مختلف الحديث) is where two sound hadiths seem to clash and the compiler resolves it - al-Tirmidhi does this more than al-Bukhari, and both do it sparingly. Alongside it, rulings drawn from a hadith outside the chapter heading; Abu Dawud does that most, then al-Tirmidhi.