Tsi writes the affricate ts and carries no numeral value in the Glagolitic system, standing past the end of the counted series. It is one of the Slavic letters at the far end of the order, and it records a sound that Greek did not have and could not have supplied a sign for.
The affricate arose in Slavic through a softening of an original velar before certain vowels, and its distribution is therefore regular rather than arbitrary. A reader who knows the alternation can predict where it will appear, and can see the same root behind two spellings that look unconnected. That regularity is one of the reasons the second half of the Slavonic alphabet looks as it does: the letters there are not a miscellany but a set of signs for the outputs of a small number of sound changes.
The letter's name is opaque, and the reference literature records it without an etymology, along with the other names at this end of the alphabet.
Its shape is one of the Glagolitic forms most often pointed to by the scholars who look for Semitic models for the script, and the atlas records that proposal as a debated relation rather than as a finding.
Like every letter in this stretch of the order it carries no number, because the Glagolitic count runs through the letters that answer to Greek ones and stops. Cyrillic, working differently, does give its own letter for this sound a value, fitting it into a Greek slot that had fallen vacant, so the same sound is numbered in one Slavonic script and unnumbered in the other.