The ten letters at the end of the traditional Mongolian alphabet exist because the script was asked to write words it was not built for. Chinese contributed vocabulary with sounds and syllables Mongolian lacks; Tibetan and Sanskrit contributed the entire technical vocabulary of Buddhism, with distinctions of aspiration and articulation that Mongolian does not make. The response was to extend the alphabet rather than to distort the words, which is the choice that produced these letters: an f, two further velars, an aspirate, a dental affricate, a voiced sibilant, a cluster sign, a letter for the Tibetan voiceless lateral, and two letters for Chinese syllables. Read as a group they are a fairly precise record of which foreign vocabulary mattered enough to be written accurately.
Beyond them lies a further apparatus, encoded in Unicode under the name Ali Gali, for writing Sanskrit and Tibetan in full rather than approximately, which belongs to the same effort at a greater level of ambition. Neither set is a letter of the Mongolian language, and the alphabet is better understood with the seam between the inherited twenty-five and these additions kept clearly in view. Their shapes follow the script's usual habit of adapting what is already present rather than inventing, so most of them are recognisably modifications of inherited letters, which is also why they sit comfortably on the same spine and in the same hand.