Aq is the nineteenth letter of the Mandaic alphabet and writes q, a voiceless stop made at the very back of the mouth, further back than k. It joins on both sides and takes initial, medial, final and isolated shapes according to where it falls in a connected run, so it never forces a break in the middle of a word.
Its name follows the alphabet's usual pattern of the sound with a preceding a, and the back of the head that stands behind the Syriac and Hebrew names has no counterpart in it.
Nineteenth place is where qoph stands in the inherited Aramaic sequence. In Hebrew and in Syriac the letter at that place is the first of the four that carry the hundreds, and in the Arabic abjad reckoning it carries one hundred as well. In Mandaic it carries nothing of the kind. No comparable use of the Mandaic letters as figures is described in the reference literature, and the modern character standard encodes no Mandaic digits at all.
That absence is recorded in this atlas as an open question rather than as a settled fact, because nothing states that the practice was absent either. It is a gap in the record and not a disagreement between scholars.
Because aq joins on both sides it never forces a break in the middle of a word, unlike halqa, az, it, aksa and ash. Its counterparts in Syriac and in Arabic behave the same way, so the nineteenth place happens to be one where the joining behaviour agrees across the three scripts, which is not the general rule.