The title of zan is a Klingon civilization concept of address in the sense that the word "mister" is used among Human military and service officers. The address as zan is usually invoked by a Klingon military captain or commander addressing a most junior subordinate, typically a junior of officer rank, such as an ensign or lieutenant. (TOS - Worlds Apart novels: The Final Reflection, How Much for Just the Planet?; TOS novel: Rules of Engagement)