The big yus stands thirty eighth and writes the nasal back vowel, the second of the two nasals of Old Church Slavonic. It carries no numeral value.
Its shape is the most elaborate in the alphabet, and its name is a Slavist's label distinguishing it by size from the front nasal rather than a word of the language.
Its history is the most interesting of the four yus letters because it did not simply disappear. In Bulgarian the nasal quality of the vowel was lost but the vowel itself was not: it developed into a distinct non-nasal sound, and the letter went on being written for it. That kept the big yus in active daily use in a national orthography until 1945, roughly a thousand years after the sound it was made for had ceased to exist anywhere.
Elsewhere it went with the nasals. Russian lost it, Serbian lost it, and no modern alphabet outside the Bulgarian case retained it.
The Bulgarian episode is the subject of the companion article, and it is worth attention because it is the clearest example in this alphabet of a letter surviving by changing what it means rather than by being useful for what it originally said.
The Serbian recension shows the other way a letter can outlive its sound. By the time Serbian manuscripts begin, the back nasal had already resolved into a plain u in that area, so a Serbian scribe writing the big yus was following a spelling convention rather than recording anything he said. Where the letter appears in such a manuscript it is evidence about the exemplar being copied and about the training of the copyist, and it is no evidence at all about the language of the place the copy was made in. Distinguishing those two kinds of testimony is most of the work in dating a Slavonic hand.