COI (http://www.lojban.org/en/publications/reference_grammar/chapter6.html, http://www.lojban.org/en/publications/reference_grammar/chapter13.html)

When prefixed to a name, description, or sumti, produces a vocative: a phrase
which indicates who is being spoken to (or who is speaking). Vocatives are used in
conversational protocols, including greeting, farewell, and radio communication.
See DOI, DOhU.

coi .djan. Greetings, John.

