Types of letters

Res privatæ

Ex officio

(written by some person acting in an official capacity, like safe-conducts, notifications, injunctions, etc.)

Arquivo Nacional Torre do Tombo - Correspondência de soberanos.

Ex labore

(written by a professional, concerning matters of his art, science or expertise)

Ex negotio

(business correspondence)

Ex opere operato

(epistolary acts: testaments, letters of credence, war declarations, etc.)