Ain is the last sign in the Mandaic range of the modern character standard, and it is a borrowing from the Arabic letter of that name, taken in for the sounds of Arabic words. It joins only to the right.
It is not part of the inherited Semitic sequence and it is not one of the traditional twenty-four. The alphabet's own sixteenth letter, in, is the inherited descendant of Aramaic ayin, and the two should not be confused; the atlas records a relation between them marking them as distinct for exactly that reason.
The standard records ain at the end of the Mandaic range, after the two ligature letters, which is where a late addition naturally falls in a list arranged by the alphabet's own order.
Its presence records what centuries of contact did to a small liturgical language surrounded by Arabic. Mandaic answered most such pressure by marking its inherited letters rather than adding new ones, so a borrowed letter is the exception, and it is worth noticing which sound was worth the exception: the Arabic voiced pharyngeal, in a language whose own pharyngeals had weakened away.
Because ain joins only to the right, the letter after it begins a fresh connected group, which it shares with the two ligature signs that precede it in the range. All three of the signs following the inherited twenty-two behave the same way on the page, so the additions to the Mandaic alphabet are visible in how a line breaks as well as in where they fall in a list.