Slovo writes the consonant s and takes the numeral two hundred. Its name is the Slavonic word for word, and of all the letter names it is the one that carries the most weight in the tradition, because the opening of the Gospel of John, which the mission's translators rendered into Slavonic, begins with that word. A letter of the alphabet made for the translation of scripture is named with the noun that stands at the head of the text it was made to carry.
The sound is a plain sibilant. It is common, it is stable, and it takes part in the alternations that produced the palatal fricative earlier in the alphabet, so a reader meets the same root spelled two ways in related forms.
The numeral two hundred follows from counting the Glagolitic letters in order, this being the twentieth sign.
The name also completes the clause that most readings of the recitation accept: say the word firmly. Whether the letter names beyond this point continue that text, and whether the whole sequence was composed as a connected utterance at all, is a scholarly question rather than a settled one, and the atlas treats it that way.
Slovo is also one of the letters that the Slavonic tradition kept in continuous use as a numeral long after Glagolitic itself had been displaced. Two hundred is a common figure in a psalter or a gospel book, and the practice of numbering with letters outlasted the change of script, so a Cyrillic manuscript centuries later is still doing with its own slovo exactly what a Glagolitic scribe did with this one. The habit is older than either alphabet, having come to the Slavs from Greek along with the books.