Zha is the twenty-first of the thirty consonants and opens the sixth line. In the classical language it wrote a voiced palato-alveolar fricative, the s of English measure. Modern Lhasa speech devoices it toward the sh of ship and marks the difference from sha with low tone instead of with voicing, which is the trade the whole voiced series of the alphabet has made.
Zha and sha are a pair, and they stand to sa and za as the palatals stand to the dentals. The recitation makes the symmetry audible, and it is one of the small consolations of the sixth and seventh lines, which are otherwise the least orderly part of the alphabet.
The syllable zha is also the base of a small family of very common honorific words, so a reader meets the letter early and often. Tibetan maintains a distinct honorific vocabulary, a separate set of words used of a respected person and of things belonging to one, and several of the most frequent of those words begin here. That is a fact about the lexicon rather than about the letter, but it is why the shape becomes familiar quickly.
Zha takes superscripts and the wa subscript, and it plays no part itself in the prefix, suffix, superscript or subscript systems. That is the ordinary condition of the letters in the last three lines, with the single exception of ra, la and sa, which between them carry almost the whole stacking apparatus.