Sa is the twenty-eighth of the thirty consonants and closes the seventh line. It writes a voiceless alveolar fricative, the s of English see, and it carries high tone as an initial in modern Lhasa speech.
Like ra and la it is one of the busiest letters in the script. It is one of the three superscripts, one of the ten suffixes, and, uniquely with the classical da, one of the two letters that may stand in the second suffix slot after another final consonant. Only ra matches it for the number of positions it can occupy.
As a superscript, the sa mgo, it is silent in modern speech and its former presence is carried by the tone of the syllable beneath it. As a suffix it closes the syllable in the ordinary way. As a second suffix it is doing something different in kind, because in that position it is very often grammar rather than lexicon.
Sa is also the voiceless counterpart of za, distinguished from it in the classical language by voicing and in modern Lhasa speech by tone, which is the same trade the whole voiced series has made.
The one office sa lacks is the prefix. That leaves ra and sa as the two letters appearing in the greatest number of positions in the script, and between them they account for a good deal of what makes a Tibetan syllable look complicated on the page and read straightforwardly once the slots are known.