Sha is the twenty-seventh of the thirty consonants and writes a voiceless palato-alveolar fricative, the sh of English ship. It carries high tone as an initial in modern Lhasa speech.
It is the voiceless counterpart of zha in the sixth line, and the pair sha and zha stands to sa and za as the palatals stand to the dentals, which is a symmetry the recitation makes audible even though the lines are otherwise not organised by place.
Sha sits in the seventh line between ra and la on one side and sa on the other, and it is unlike its neighbours in a way worth noting. Ra, la and sa carry almost the whole weight of the Tibetan stacking system between them, appearing as superscripts, subscripts and suffixes. Sha does none of that. It is a root letter only, and appears in no prefix, suffix, superscript or subscript position, so it shares a line with the three busiest letters in the alphabet and has no office at all.
That is a reminder that the lines of the recitation are containers rather than categories once the first four are past.
Sha takes superscripts and the wa subscript. In the transcription of Sanskrit it also has a reversed counterpart, used for the retroflex sibilant that Sanskrit distinguishes and Tibetan does not, so the letter is one of the small set that has a mirror image standing outside the alphabet.