Za is the twenty-second of the thirty consonants and, in the classical language, the voiced alveolar fricative, the z of English zoo. In modern Lhasa speech it is devoiced toward s and is distinguished from sa by low tone rather than by voicing.
Za is the plainest single illustration of the largest sound change between the language the alphabet was designed for and the language spoken in Lhasa today. Two letters that once differed in the vocal cords now differ in the pitch of the voice, and no distinction has been lost, only moved. The spelling has not followed the change, which is why written Tibetan looks so much more conservative than spoken Tibetan and why the tone of a syllable is largely predictable from how it is written.
The change is not universal across the Tibetan-speaking world, and the article beside this one takes up what that means.
Za takes superscripts and the wa subscript. It is not a prefix, not a suffix, not a superscript and not a subscript, so it belongs entirely to the recitation and to the root of a syllable.
As an initial it carries low tone in modern Lhasa speech, in company with the other letters that were voiced in the classical language. The pair za and sa is one of the clearest places to hear what the tone system is doing, since two syllables built on them can differ in no consonant and no vowel and be told apart by pitch alone.