While discussing the various forms of documents issued by the pope (e.g., Moto Proprio) A. Burt Horsley, a Latter-day Saint who was an observer at Vatican II, wrote the following which is rather helpful for those unfamiliar with such term and documents:
The question might well be posed, through what media is the voice of Peter heard? Just how do the various popes make their pronouncements to the world and in what manner is their word—or the will of Peter—given out for all to know? Among the earliest sources would be the epistles to which reference has already been made, accredited to Peter himself. Linus did not write anything of which we have record, and there is nothing attributed to Anacletus. But the church gives considerable attention to the writings of Clement, which is well accepted and perhaps one of the finest extracanonical documents we have from this period, is considered very valuable by the Catholic church in helping to establish authority. At a later time, epistles were used frequently by the bishops of Rome to manifest their will unto the world.
There are also other official documents of papa origin which have come down to us through tradition, taking precedence as media through which the pope makes the will and position of the church known not only to the church, but to the world at large.
One of the most frequently used in times past is called the papal constitution. There had been very little attempt to distinguish the constitution in its unique usage from among other pronouncements or documents until the year 1587 when Sixtus V in his Papal Bull Immensa eterni precisely defined the various writings and announcements that proceed from the Vatican and described the particular format identified with each one according to its purpose.
Papal constitutions are ordinarily used for doctrinal disciplinary pronouncements. A papal bull is a device by which dignitaries are appointed, dioceses are erected and saints canonized. However, prior to the year 1587 a papal bull might have been used for other purposes. For example, it was a papal bull which officially excommunicated Luther and put him under the ban of the church. And it was this document that Luther burned publicly before the Elster gate in Wittenberg along with the canon law, to show his contempt.
There is the papal brief. This generally grants less important decrees or privileges and concessions. The encyclicals, which are perhaps the most important of the forms in terms of discovering doctrine, along with the papal constitution, are papal letters relating to doctrinal or moral matters, exhortations, warning, or commendations. The word encyclical means that it is a latter which is of a circular nature: it is circulated among all the bishops of the church so that all are aware of its contents.
A rescript regularly contains some grant or favor, some dispensation or privilege bestowed in consequence of a previous petition or request.
There are also decrees, or papal instructions ordinarily issued by one of the Roman offices or congregations in the name of the pope to which the pope’s approval is attached either in forma communi, the common form; or in forma specifica, the special form.
Finally, there are two other types of Letters Apostolic that are not usually significant for doctrinal purposes; Moto Proprio, an action taken on the pope’s personal initiative, and the Chirographi, autograph letters written in the pope’s own handwriting. (A. Burt Horsley, Peter and the Popes [Religious Studies Center Specialized Monograph Series vol. 5; Provo, Utah: Religious Studies Center Brigham Young University, 1989], 105-7, italics in original)
