Small yus with tail is a variant form of the letter for the front nasal vowel, encoded in the Unicode Standard under that name, and it carries no numeral value.
Variants of this kind are ordinary in a manuscript script. A scribe writing by hand over a long text produces forms that differ according to position, according to the letters on either side, and according to how the shape was taught in the tradition he learned in, and when a variant becomes regular enough it gets recorded as a distinct form. Whether it is a distinct letter is a different question, and the answer generally is that it is not.
That is why the sign carries no number and does not add to the count of the alphabet in any of the traditional reckonings. The counted series runs through the ordinary letters and stops well before this part of the order in any case, and the recited alphabet names the letter once rather than twice.
The atlas lists it because it lists what the Unicode Standard encodes, and because a form a scribe used is worth a record. It says plainly that the sign is a variant so that no reader takes the length of the list as the length of the alphabet.
Nor does the atlas pair the sign with a Cyrillic letter. Cyrillic has a letter for the front nasal vowel and the atlas pairs it with the ordinary Glagolitic sign, which is the letter the variant varies from. Adding a second pairing for the variant would suggest a distinction in the Cyrillic inventory that does not exist.