Da is the eleventh of the thirty consonants and the voiced member of the dental line, a voiced d in the classical language and an unaspirated dental with low tone in modern Lhasa speech.
It carries more orthographic work than almost any other letter in the script. It is one of the five prefixes, standing in front of a root letter without being pronounced. It is one of the ten suffixes, closing a syllable. And in the classical language it was also the second suffix, a d written after another final consonant, known as the da drag or strong da. Three offices, none of them audible in the modern standard language, all of them written.
The consequence for a reader is that a da in a Tibetan syllable has to be placed before it can be read. Is it the root letter, or is it standing in front of one? Is it closing the syllable, or is it sitting after the letter that closes it? The answer determines the pronunciation, the tone and the meaning, and it is settled by knowing which letters may occupy which slots rather than by anything in the shape of the da itself.
This is the general condition of Tibetan writing rather than a peculiarity of one letter, but da is where it is most concentrated, and a grammar demonstrating the slot system will very often demonstrate it with da.