Question: How does one know whether to refer to a papal document as a bull, an encyclical, an apostolic letter, a motu proprio, or whatever?
Answer: The system is rather complex, with documents generally being classified based on who is being addressed, and what is the pope’s motivation for writing. Some of this has changed over time. Some of the terms are ambiguous. A brief summary follows but the interested reader is invited to consult the Catholic Encyclopedia article s.v. “Bulls and Briefs.”
Apostolic Constitutions address all of the faithful, and address some matter of faith or morals, or a universal discipline. A Constitution is the likely document to contain an infallible pronouncement of faith or morals, for such pronouncements are always universal; addressed to all Christians and not to some individual or group, for truth is universal.
Encyclicals are circular letters, traditionally addressed to some or all of the bishops and prelates in communion with the Apostolic See. In recent years, the address has been much wider.
Motu proprio describes an apostolic letter written at the Pope’s own initiative.
A Bull is a letter from the Pope or papal chancery which gets its name from the heavy lead authentication seal attached to it. Since the pontificate of Pope Leo XIII, most bulls are now sealed in simple red ink.
Briefs are less formal than Bulls, both in style and seal. The distinction has been blurred since Leo XIII.
A Chirograph bears the handwritten signature of the Pope. The term is sometimes applied to handwritten drafts in the early stages of developing a formal document.
Decretals are answers to some doubt or difficulty submitted for judgment to the Holy See, applicable not only to the petitioner, but having the force of general law.
Rescripts also answer doubts or difficulties submitted to the Holy See, but their answers bear only on the original petitioners.