Zhivete writes the consonant zh, a voiced palatal fricative, and takes the numeral seven. It is one of the letters that exist only because Slavonic needed them: Greek has no such sound and no letter for it, so the sign has no Greek counterpart anywhere in the order and no Greek numeral value to inherit.
The sound arose in Slavic through a change that softened certain consonants before front vowels, and its distribution in Old Church Slavonic is therefore not random. It turns up in particular grammatical positions, in verb forms and in derived words, where an original velar has been softened, and a reader who knows the pattern can often predict where the letter will appear. That regularity makes it useful in comparative work, because the same alternation runs through the modern Slavic languages in the same places.
Its name is the plural imperative of the verb to live, live, and it carries the recited alphabet into its third clause. The Slavonic sentence built from the letter names moves from I know letters and speech is good into a passage that has been read in more than one way, and zhivete is where the plain reading begins to be harder to sustain.
The numeral seven comes from the Glagolitic practice of counting its own letters in order, which is the only system available for a letter with no Greek equivalent.