Sha is the palatal sibilant and the first of the three that open the last block of the consonant order. Sanskrit has three sibilants where most of the languages it is related to kept one, and the distinctions between them are held apart by rule rather than left to the ear, so a script that writes Sanskrit needs all three letters and Siddham has them.
The letter stands at the head of a large family of religious vocabulary, including the words for peace and for the auspicious and a great many names built on them, and it is frequent in the Buddhist texts copied in Siddham.
It enters conjuncts readily, and several of its combinations, particularly with the semivowels and with the stops of its own class, are common enough to have settled into shapes a learner acquires as units.
Its place in the order marks a transition. The twenty-five stops are behind it and the four semivowels are behind it, and what remains after this letter is the retroflex sibilant, the dental sibilant and the aspirate: four letters, and then the table closes.
The three sibilants also mark the point where the table stops being a grid. The twenty-five stops are a crossing of five places against five treatments, and the four semivowels are a block paired to four vowels, but the sibilants and the aspirate are a short closing list, classified by place and by nothing else. A learner reciting the order feels the change: the pattern that carried the first two thirds of the table runs out, and the last four letters have to be held individually.