Iotated small yus writes a glide followed by the front nasal vowel, and it carries no numeral value. It combines two of the features that make the tail of the Slavonic alphabet what it is: the nasal vowels the language inherited, and the treatment of a glide plus vowel as a single unit deserving a sign of its own.
The result is a letter for a sound sequence that no Mediterranean script had any use for, and it is one of the plainest demonstrations that the alphabet was fitted to Slavonic rather than borrowed and made do with. A designer working from Greek alone could not have arrived at it.
The sequence is not frequent, and the letter is correspondingly less well attested than the plain nasal vowel signs. Where it does appear it is usually in particular grammatical endings, which is the general pattern for the glide sequences: they cluster in the inflectional system rather than spreading evenly through the vocabulary.
The name is opaque like the others at this end of the alphabet, and the atlas records that gap as an open question about the whole set of Slavonic letter names rather than guessing at any single one.
Nothing in the Glagolitic numeral system reaches this far, so the letter is unnumbered, as every sign after the second o letter is. The Cyrillic counterpart is unnumbered too, for the different reason that Greek had no letter here to lend a value.