This letter stands thirty seventh and writes a glide followed by the nasal front vowel. It carries no numeral value.
It is built on the little yus with the glide sign in front of it, which puts it in the same class as the iotified a and the iotified e: a compound whose parts remain visible, counted as a letter in some alphabet lists and as a spelling in others.
The sequence it writes is a real one in Old Church Slavonic, occurring at the start of words and after vowels where the plain little yus occurs after consonants, the same distribution that governs the whole iotified series. What makes this letter and its companion in the big yus series difficult is that the sequences are uncommon, so the letters are uncommon, and a sign that appears rarely is a sign scribes copy carelessly.
The manuscripts confirm it. Confusion between the iotified and plain yus letters begins early and is widespread, which means the evidence for how this letter was written, and in some cases for whether a given text used it at all, is thinner than for any other part of the alphabet.
Nothing of it survives in a modern alphabet. The nasal vowel it depended on was lost across almost the whole Slavic area, and a letter whose sound has gone and whose shape was already unstable has very little to hold on with.