Yeri, in the arrangement the atlas follows, writes the vowel conventionally transliterated y, a high back unrounded vowel distinct from the ordinary i, and it carries no numeral value.
The vowel is one of the more distinctive sounds of the Slavic languages and it survives in several of them. In Old Church Slavonic it stands in regular alternation with i, appearing after certain consonants where i appears after others, so a reader who knows the pattern can often predict which of the two a given ending will show.
How the earliest Glagolitic wrote this vowel is described differently by different authorities. Some accounts treat it as a compound, written by placing two existing signs together, and others list it as a letter in its own right. The Unicode Standard encodes a letter here, which is the arrangement the atlas follows, and the disagreement in the literature is about the manuscripts rather than about the encoding.
The name is opaque in both Slavonic scripts. That is the ordinary condition of the letter names in this part of the alphabet, and the atlas carries an open question about the whole set rather than proposing readings for individual names.
No numeral attaches to the letter in either Slavonic script. It stands past the end of the Glagolitic counted series, and Cyrillic, numbering only what Greek numbered, had nothing to give a vowel that Greek did not possess. The unnumbered tail of the alphabet is where the two scripts are most alike in what they were trying to do and least alike in how they look doing it.