This letter can be found in the appendix to the Norberg's edition of Gregory's letters. It was sent in all likelyhood shortly after John of Ravenna had received an epistle from Gregory dated to April 593 and before the next of Gregory's letters, dated to October 594.
This letter demonstrates that it was customary for the bishops of Ravenna to be ordained in Rome, and that their ordination was witnessed by the presbyters of this see.
It is far from clear what a mappula was. Du Cange (s.v. "Mappulas") considers it a piece of an ornamental garment used particulary by the Roman clergy. This letter, along with the Gregory the Great's letter to which John of Ravenna here responds (see [2349]), suggest that it was worn particulary during official processions. Martyn, Carole Straw (1988: 82), and Thomas F.X. Noble (2017: 51) assert, further, that this was a "linen saddle-cloth", but they do not point to the source of this claim.
It is also possible that mappula is to be identified with a maniple.