Ra is the twenty-fifth of the thirty consonants and opens the seventh line. It writes an alveolar trill or tap, and it carries more structural work than any other letter in the script.
Ra is one of the three superscripts, one of the four subscripts, and one of the ten suffixes. It can therefore appear above a root letter, below one, after one, or as the root itself, and a reader has to determine which before the syllable can be read. No other letter occupies four positions.
Written above a root letter it takes a shortened form, and in modern Lhasa speech it is silent, its former presence carried by the tone of the syllable beneath it. Written below, it alters the consonant above it rather than sounding. Written as a suffix, it closes the syllable and affects the vowel in front of it.
Because the superscript form is shortened, digital text needs a way to prevent the ordinary shape from combining when a plain ra is wanted next to something it would otherwise stack with. Unicode provides a separate fixed-form character for exactly that case, which is a small example of a very old scribal distinction surviving into an encoding standard.
What ra is not is a prefix. The five prefixes are ga, da, ba, ma and the small a, and ra is absent from that list, so of the positions available in a Tibetan syllable it fills every one except the first.