Ksi stands fortieth, writes the cluster ks, and carries the numeral 60 from Greek xi.
Slavonic did not need it. The cluster it writes could be spelled perfectly well with the letters for k and s, and in ordinary Slavic vocabulary it is. The letter exists because Greek had it, because the Greek numeral series needs a sign at 60, and because a very large part of the vocabulary of the church books is Greek names and Greek technical terms whose spelling the tradition preserved.
That gives it a peculiar status. It is a letter of the alphabet in the sense that it appears in the alphabet lists, has a name and carries a number, and it is not a letter of the language in the sense that no Slavonic word requires it. A scribe could go a long way through an ordinary text without writing one.
Its numeral function was the more important of its two jobs and the one that kept it alive longest. Sixty is needed, and no other letter could supply it.
It was among the letters removed from secular printing when a civil type was introduced in the early eighteenth century, and it survives today only in the church books, where the old numeration is still in use and the Greek spellings are still preserved.