It is transparent just as a ghost, so it is some kind of "ghostletter"
When the definitions of beasts versus beings were first being debated, ghosts and other spirits felt distinctly put off by the constant talking of needs of the living over desires of the dead. When the issues were finally resolved, and there were made three classes, beast being and spirit, its likely that the spirit division of the ministry of magic department for the control and regulation of magical creatures set to work inventing many spectral items for ghostly use. Ways to create papers, quills, books, and other such necessities for ghosts to use. Professor Cuthbert Binns is another example of a ghost who must, logically, use spectral items, otherwise he would not be able to turn the pages of his own notes.