Pe is one of the least secure letters in the Glagolitic repertoire. The Unicode Standard encodes it and gives it that name, placing it in the order after the second sign for the vowel o, and the reference literature reads it as a form of p without settling what distinguished it from the ordinary letter for that sound.
Signs of this kind reach us mainly through abecedaria, the lists of the alphabet copied out as exercises, as inscriptions or at the head of a book, rather than through running text. That is a weak kind of attestation. An abecedary tells you that a scribe knew of a sign and thought it belonged in the list; it does not tell you what the sign was for, how it was pronounced, or whether the scribe had ever used it to write a word.
The letter carries no numeral value in the atlas, and that is a deliberate record of uncertainty rather than a statement that it had none. Different sources place different values in this part of the order, and choosing among them would mean asserting one reading of a disputed slot.
The atlas therefore lists the letter, states plainly that its value is not established, and carries an open question saying what would settle it. That is a better outcome than either omitting the sign or filling the gap with a plausible guess.