The Pericardium's clearest textual role is structural: Chinese medical theory organizes the body around twelve primary channels, six yin and six yang, and without the Pericardium and its paired yang partner, the Triple Burner, the system would only reach ten. Adding this pair keeps the yin and yang channels symmetrical and lets each of the twelve two-hour periods of the Chinese clock be assigned its own channel, the Pericardium governing the period from roughly seven to nine in the evening.
Later systematizers of the theory, working from the Yellow Emperor's Inner Classic and subsequent commentary, treat the Pericardium-Triple Burner pair as functional complements to the Heart-Small Intestine pair rather than as organs on equal independent footing with the other five zang-fu pairs, a distinction still taught in contemporary Chinese medicine education.
There is textual evidence that the twelfth channel really was an addition rather than an original member of the set. The medical manuscripts recovered from the Mawangdui tombs, which predate the received Inner Classic, describe eleven vessels rather than twelve, and the one they do not describe is the arm channel later assigned to the Pericardium. The symmetry of the twelve channel system, in other words, appears to have been arrived at rather than inherited.
That history sits well with the Pericardium's ambiguous standing in the finished scheme: a channel with a full complement of points and a place on the clock, attached to an organ that five phase theory never found a slot for.