Yat stands thirty second and writes the vowel Slavists call yat. It carries no numeral value.
It is the letter that most rewards attention, because it never had a single agreed pronunciation. From the earliest period its value differed by region: something like a long front e in some areas, something closer to ya in others, and intermediate values elsewhere. That variation is not later corruption. It is present in the record from the beginning, and it is one of the criteria by which the recensions of Church Slavonic are distinguished.
The consequence for scribes was severe. A letter whose sound you cannot hear as distinct from another letter's sound is a letter you can only write correctly by knowing which words take it, and the confusions in the manuscripts are accordingly frequent and informative. A Slavist can often place and date a manuscript by watching where its scribe writes yat and where he slips.
In modern languages its reflexes diverge completely: Russian merged it with e, Ukrainian with i, and Bulgarian has both outcomes in different parts of the country, divided by a line running roughly north to south that has its own name in Bulgarian dialectology.
It survived in Russian orthography until 1918 and in Bulgarian until 1945, in both cases centuries after it had stopped marking anything a speaker could hear.