Right out of your own flippin' mouth, hombré.
By the accession of Pope Leo IX in 1048, a clear distinction developed between two classes of bulls of greater and less solemnity.
Papal bulls were originally issued by the pope for many kinds of communication of a public nature, but by the 13th century, papal bulls were only used for the most formal or solemn of occasions.
Today, the bull is the only written communication in which the pope will refer to himself as "Episcopus Servus Servorum Dei" ("Bishop, Servant of the Servants of God").[3] For example, when Pope Benedict XVI issued a decree in bull form, he began the document with "Benedictus, Episcopus, Servus Servorum Dei".
In terms of content, the bull is simply the format in which a decree of the pope appears. Any subject may be treated in a bull, and many were and are, including statutory decrees, episcopal appointments, dispensations, excommunications, Apostolic constitutions, canonizations, and convocations.
The bull was the exclusive letter format from the Vatican until the 14th century, when the papal brief appeared.