To write Sanskrit retroflexes, the sounds made with the tongue curled back which Sanskrit distinguishes and Tibetan does not have, Tibetan uses mirror images of its own dental letters. Ta reversed writes the retroflex, and so in turn do reversed tha, da, na and sha. They are known as the log yig, the reversed letters.
The device is economical and it is honest about what it is doing. It says, in effect, that this is the dental letter and something has been done to it, which is exactly the relationship the two series have. It also keeps the reversed forms visibly outside the alphabet proper: a reader can see at a glance that a word containing one is a transcription rather than a Tibetan word, in the same way that italics mark a foreign term in an English text.
They are not part of the sum cu. A Tibetan child recites thirty letters and does not recite these, and they are taught later and to a narrower group, since a reader of ordinary Tibetan will meet them chiefly in mantras, in Sanskrit names and in the technical vocabulary of Buddhist scholastic texts.
They are encoded separately in Unicode as well, which reflects the same judgement: the reversed letter is its own character, not a formatting variant of the letter it mirrors, because the two are contrastive and a search that conflated them would fail.