Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon VIII

p. 112 Canon VIII.

Let not country presbyters give letters canonical, or let them send such letters only to the neighbouring bishops.  But the chorepiscopi of good report may give letters pacifical.

Notes.

Ancient Epitome of Canon VIII.

A country presbyter is not to give canonical letters, or [at most] only to a neighbouring bishop.

These “letters canonical” were called in the West letters “formatæ,” and no greater proof of the great influence they had in the early days of the Church in binding the faithful together can be found than the fact that Julian the Apostate made an attempt to introduce something similar among the pagans of his empire.

“Commendatory letters” (ἐπιστολαὶ συστατικαὶ) are spoken of by St. Paul in 2 Cor. iii. 1, and the reader will find some interesting remarks on this and cognate subjects in J. J. Blunt’s, The Christian Church during the first three Centuries (Chapter II).

By means of these letters even the lay people found hospitality and care in every part of the world, and it was thrown up against the Donatists as a mark of their being schismatics that their canonical letters were good only among themselves.

Pseudo-Isidore informs us that it was stated at the Council of Chalcedon by Atticus, bishop of Constantinople, that it was agreed at the Council of Nice that all such letters should be marked Π. Υ. Α. Π. (i.e. Father, Son, Holy Spirit), and it is asserted (Herzog, Real-Encyk., s.v. Literæ Formatæ) that this form is found in German documents of the sixth century.

As will be seen among the Canons of Chalcedon, the old name, Letters Commendatory, is continued, but in this canon and in the 41st of Laodicea the expression “Canonical Letters” is used.  In the West, at least, these letters received the episcopal seal of the diocese to avoid all possibility of imposture.  Dean Plumptre (whom I am following very closely in this note) believes the earliest evidence of this use of the diocesan seal is in Augustine (Epist. lix. al. ccxvij.)  He also refers to Ducange, s.v. Formatæ.

As these letters admitted their bearers to communion they were sometimes called “Communion letters” (κοινωνικαὶ ), and are so described by St. Cyril of Alexandria; and by the Council of Elvira (canon xxv.), and by St. Augustine (Epist. xliii. al. clxii).

The “Letters Pacifical” appear to have been of an eleemosynary character, so that the bearers of them obtained bodily help.  Chalcedon in its eleventh canon ordains these “Letters pacifical” shall be given to the poor, whether they be clerics or laics.  The same expression is used in the preceding canon of the synod.

A later form of ecclesiastical letter is that with which we are so familiar, the “letter dimissory.”  This expression first occurs in Canon XVII. of the Council in Trullo.  On this expression Suicer (Thesaurus, s.v. ἀπολυτικὴ) draws from the context the conclusion that “letters dimissory” were given only for permanent change of ecclesiastical residence, while, “letters commendatory” were given to those whose absence from their diocese was only temporary.