E is the sixteenth letter of the Syriac abjad, with the numeral value seventy. It writes a voiced pharyngeal, the second of the two pharyngeals that Syriac keeps from its Semitic inheritance, the other being heth at the eighth place. It joins on both sides.
It stands in the sequence where Hebrew has ayin, whose name it continues, and its name is the inherited Northwest Semitic word for an eye.
The oddity here is the name as it appears in this atlas and in character standards generally. English descriptions transliterate the Syriac name in several ways, none of which is wrong and none of which has become the single convention, and the modern character standard settled on recording the letter simply as E. That is why the sixteenth letter of a Semitic abjad can turn up in a list of letter names looking like a Latin vowel, which it is not and never was.
The sound is worth being clear about too. A voiced pharyngeal has no counterpart in most European languages, and it is not a vowel, not silent, and not a glottal stop. In the classical reading it is a consonant made deep in the throat. In several of the modern spoken descendants of Syriac the pharyngeals have weakened, so the letter is written where the classical spelling calls for it and read according to the tradition of the community reading it.