The Jami of al-Tirmidhi is arranged by legal chapter like the other collections and does something none of them does consistently. After most reports he adds a note: which companions transmitted something similar, which jurists in which cities acted on it, whether the schools divided over it, and what he himself thinks of the chain. The book therefore reads as a compilation with a commentary already attached, and a reader gets not only the report but its standing and its use.
He is credited with putting the grade hasan, good, into general circulation as a term of art. Before it, criticism worked mostly with a strong contrast between sound and weak, which left a large body of material that jurists relied on with no honest label. A middle term made it possible to say that a report is not of the highest grade, is nonetheless supported, and may be acted on, which is closer to how the law was actually being reasoned. The closing section of the book, on the defects of chains, sets out his criteria directly, so the working method is recoverable rather than inferred.
He had studied with al-Bukhari and cites him by name when reporting a judgement on a chain, which gives the book a second use as a record of what the most exacting critic of the preceding generation had said about particular reports. Some of those opinions survive nowhere else. A collection that preserves its teacher verdicts alongside its own is doing the work of an archive as well as of a reference, and that is one reason this book has stayed in use for eleven centuries.