Ssa is the retroflex sibilant, made with the tongue tip curled back, and the second of the three sibilants. In classical Sanskrit it is largely produced by rule: a dental sibilant converts to this one after certain vowels and consonants, so its distribution is predictable to a reader who knows the grammar and unpredictable to one who does not.
The letter is common in Buddhist vocabulary and joins freely into conjuncts. Its cluster with the velar stop, which writes the sound conventionally transliterated ksh, is among the most frequent in the language and is treated in several Indian traditions almost as a letter in its own right, with a shape that has to be learned rather than derived.
It stands between the palatal and the dental sibilant in an order that runs from the palate to the teeth, which is the same principle that orders the five classes of stops and the semivowels before them.
The letter also appears in a great many of the syllable strings transmitted for their sound rather than their sense, where its correct identification matters more than usual, because nothing in the context can restore it if a copyist gets it wrong.
Its predictability by rule makes it a good test of whether a copyist understood what was being copied. A scribe who knows the grammar writes the retroflex sibilant where the rule requires it. A scribe reproducing shapes writes whatever stood in the exemplar, including its errors. The distribution of this one letter across a manuscript can therefore say something about the hand that made it.