I'm not quite sure myself.
They say it's a series of confidential negotiations between the Masters and a devil of some note. They say it comprises the billets-doux written by Jack-of-Smiles to the Traitor Empress. They say it's the letter the Pope wrote, the one without which Rome would have been the Fourth City. They say if you read it your eyes boil and your hair turns the white of old ice. They say it's written on slate in the blood of poisoned bats.
They say it's the language the bats speak. They say the Snuffer wrote it on the outside walls of New Newgate. They say the the Topsy King learnt it, and that's why you can never understand a bloody word he says.
They say it's the mathematics of Hell. They say it's the geography of Time. They say it was invented wholesale by a honey-sipper sitting giggling in a cramped and filthy room on Hollow Street, and it's been driving gullible scholars insane ever since.
They say only the Brass Embassy knows.

"Correspondence?"