A name for the manual governing the professional standards, protocols and etiquette demanded of an Assassins' Guild member. All student Assassins are demanded to be familiar with this book and the demands it makes on them as they go about the successful completion of a contract and how they present themselves in the world at all times. This may be another name for the manual described elsewhere as the Cordati.
On Roundworld, a Concordat is an agreement or treaty, especially one between the Vatican and a secular government relating to matters of mutual interest. Was this a treaty between the ruler(s) of Ankh-Morpork and the Assassins?