Letters are a custom kind, but closer to NIP-04 as they're more for quick, sentimental communication:
https://nostrhub.io/naddr1qvzqqqrcvypzppscgyy746fhmrt0nq955z6xmf80pkvrat0yq0hpknqtd00z8z68qqg8qetj...
NIP-04 is a controlled narrative, Armada's NIP-17 DMs are just surveillance in disguise, serving the likes of Thiel and Musk.