The ars dictaminis was the medieval description of the art of prose composition, and more specifically of the writing of letters (dictamen). It is closely linked to the ars dictandi, covering the composition of documents other than letters. The standing assumption was that these writings would be composed in Latin, and according to well worked-out models. This made the arts of composition a subfield of rhetoric.
In business letters, it called for some form of address (e.g., “Worshipful master”); salutation (“I greet you well”); notification (“May it please you to know”); exposition (“the wool was shipped”); disposition (“and I want my money”); and valediction (“May God keep you well, at least until my bill is paid”). Clerks and scribes wrote the letters based on those rules.
Source: Ars dictaminis – Wikipedia