Yo writes the sequence of a glide followed by the vowel o, and it carries no numeral value. It belongs to the group of signs at the end of the Slavonic order that give single letters to combinations the language treated as units.
The sign is not common. The sequence it records is rarer in Old Church Slavonic than the corresponding sequences with u and with the nasal vowels, and a manuscript may use the letter only a handful of times or not at all. That rarity has consequences for the record: the form varies more between traditions than the forms of common letters do, and its place in the order is given slightly differently by different abecedaria.
The early Cyrillic alphabet as the atlas records it has no letter that answers to this one directly, which is why no counterpart is listed here. Cyrillic wrote glide sequences with its own set of signs, and the two inventories do not line up member for member in this part of the order. Where a counterpart cannot be identified with confidence the atlas leaves the pairing blank rather than making one, since a wrong correspondence reads on the page exactly like a right one.
No numeral attaches to the letter. It stands well past the end of the Glagolitic counted series, and there is no Greek letter behind it from which a value could have been inherited in the way the Cyrillic system inherits values, so the sign is unnumbered in every reckoning.