The sound is a velar nasal, the back of the tongue against the soft palate and the air released through the nose, which English spells ng in sing. The nasal letters form a block of eight, running from the velar nasal at the back of the mouth to the bilabial at the lips, with the palatal and the positionally restricted signs among them. Eight signs for the nasals is an unusual provision, and it exists because the recitation distinguished all eight. It stands thirty-fifth in the alphabet as a whole and nineteenth of the thirty-eight consonants, between Avestan Bhe and Avestan Ngye. That position is not arbitrary. The sequence follows a scheme, vowels before consonants and consonants by place of articulation, and each letter's place in it is a statement about the sound.
The tradition calls the script din dabireh, the religion writing. It served the Avesta and very little else, and the shape of its letter list follows the sounds of that one language rather than any general requirement of writing in Sasanian Iran. Whoever devised the alphabet is not recorded. The script can be dated and placed with reasonable confidence, but no source names the person or the group responsible, and the atlas records that absence rather than filling it. One sign to one sound is the rule this alphabet works to, and it is easier to state than to keep. Most scripts break it somewhere, usually where a sound is rare enough that no scribe could be bothered, and the interest of this one lies in how few exceptions it allows itself.