The transmission history is unusual and it is the main reason the subject can be studied at all. Two routes carried the material past the point where it dropped out of ordinary Chinese circulation.
The first is archaeological. Manuscripts recovered from the Mawangdui tombs in Hunan, sealed in the second century BCE, include bedchamber texts alongside medical, cosmological and philosophical works. Because they were buried rather than copied, they show the tradition at a date long before the compilations that later readers knew, and they were unavailable to every scholar before the excavation.
The second is Japanese. A large medical compendium assembled in Japan in the tenth century preserves a bedchamber section quoting Chinese works that no longer survive independently, which is how the contents of several lost titles are known. Later material also reached the Daoist canon, where it sits among the other technical arts.
The result is a subject reconstructed from a buried cache, a foreign compendium and a religious canon, with the ordinary manuscript tradition largely missing. The atlas records how the sources reached us because the shape of the evidence governs how confidently anything can be said, and because a reader who does not know that the record is fragmentary will read confident modern summaries of it as if they rested on a continuous tradition.