Dza is the nineteenth of the thirty consonants and the voiced member of the affricate line, written as the shape of ja carrying the tsa phru, the same small mark that gives tsa and tsha theirs. In the classical language it was a voiced dz; modern Lhasa speech devoices it and carries the difference on low tone instead.
It is the least frequent of the thirty letters in native Tibetan vocabulary. A reader can go a long way through ordinary Tibetan prose without meeting it at the head of a word, and where it does appear it is very often in a transcription of a Sanskrit name or term, standing for the Indic letter of the same value.
That gives dza an unusual position. It is a full member of the recitation, learnt in its place and written with the ordinary letters, and it is at the same time doing much of the work of the transcription apparatus, which otherwise sits outside the alphabet. The reversed letters and the subjoined ha were added for Sanskrit and stayed outside; dza was inside from the start and found itself pressed into the same service.
Like the other two affricates it is a root letter only. It can carry a superscript and the wa subscript, and it never serves as a prefix, a suffix, a superscript or a subscript itself.
Its rarity has a practical side. A learner drilling the recitation will have met tsa and tsha in ordinary words long before meeting dza in one, so of the thirty letters it is often the last to become familiar in reading rather than in reciting.