This is the epigraph of Jiangchengzi, because the first word to use this tune is called Jiangchengzi, so the name of that tune in the world is also called Jiangchengzi.
There are also many writers who wrote lyrics in the later generations, but they don't compose music, but only use the original music. Therefore, in order to let people know what kind of music is used in the words written now, it is necessary to write epigrams. It's a prefix Jiang remembers the dream he had on the 20th day of the first month. This is the name of a serious word.
If we compare the current popular songs, such as "The First Dream", if we follow the naming format of Song Ci, it will become "The First Dream on the Back of Silver Dragon", because "The First Dream" uses the song "On the Back of Silver Dragon".