Cha is the aspirated partner of the letter that heads the palatal class, and it holds the second slot exactly as the velar aspirate does in the class before it. The pair are learned together as the voiceless half of the group, and the relationship between them is one feature and nothing else.
Sanskrit uses the letter considerably less than its unaspirated partner. Its place is fixed by the shape of the table rather than by the vocabulary, which is a pattern repeated throughout the consonant section: an arrangement by articulation gives every crossing of place and feature a slot, whether or not the language fills it generously. The table is a description of the sound system, and the vocabulary distributes itself across it unevenly.
The letter behaves ordinarily on the page. It takes the full range of vowel marks, enters conjuncts, and carries the inherent short a like every other consonant sign, so standing alone it is read cha rather than ch. In the recitation of the table it is named immediately after its partner, and a learner who has the rule of the class can supply it without having seen it.
Its low frequency makes it one of the letters most likely to be miscopied in a tradition where scribes reproduce shapes rather than read words, because a hand that has written a character only a few times has no fluency to fall back on. In the East Asian material, where a great deal of copying was done by people who could not read Sanskrit, the rare letters are correspondingly the least reliable witnesses.