Alaf holds the first place in the twenty-two letter sequence the Samaritan alphabet shares with Hebrew, Aramaic and Syriac, and it takes the numeral value one from that position rather than from anything about the letter itself. The sign wrote a glottal stop in the older language, the catch heard at the front of a word that begins with a vowel. In the Samaritan reading tradition alaf is one of four letters, with iy, it and in, whose consonants have weakened, so in practice it is often sounded as a light catch or not at all, and the vowel that follows it carries the syllable on its own.
Its grammatical work is heavier than its sound. Alaf prefixes the first person singular of the imperfect verb, so it opens every I shall and I will form in the Torah. It stands at the end of a large class of words in the Aramaic the community also writes, and inside a word it can mark a vowel rather than a consonant, a role it took on precisely because its own consonant was slight.
When Greek scribes adapted the alphabet from Phoenician they had no glottal stop to write and gave the sign to the vowel a, which is how a Semitic consonant came to be the first letter of the Greek, Latin and Cyrillic alphabets. Alaf and alpha are the same inherited sign put to two different uses, and the Samaritan letter is the one that stayed with the job it was made for.
In the character standard the letter heads the Samaritan block at U+0800, under the name the community uses for it rather than the Hebrew one, and the twenty-one letters that follow run in unbroken order from there.