Halqa is the first letter of the Mandaic alphabet and writes the vowel a. It joins only to the right, so the letter after it begins a new connected group.
Mandaic is unusual among the Semitic scripts in being a true alphabet rather than an abjad. Halqa, ushenna and aksa are full letters for the vowels a, u and i, not consonant signs pressed into vowel service as they are in the Aramaic script Mandaic descends from. A Mandaic word carries its vowels in the line of letters rather than in points above and below it, which is a difference from Hebrew, Syriac and Arabic that is easy to state and hard to overstate.
Halqa is also the letter the alphabet closes on. After the twenty-third sign it is written a second time, so the sequence ends where it began, and that repetition is what brings the traditional count of the abagada to twenty-four.
Where the Mandaic shapes came from is not settled. Published accounts assign the script an ancestry either in a cursive form of Aramaic or in the Parthian chancery script, and the atlas records the disagreement rather than picking a side. What is not in dispute is the order: the first twenty-two Mandaic letters keep the inherited Aramaic sequence exactly, so halqa stands where aleph, alaph and alif stand in the neighbouring alphabets.