Null Values

There are several codes to explain different ways in which a required element's value may go missing.

  (:unac) temporarily inaccessible
  (:unal) unallowed, suppressed intentionally
  (:unap) not applicable, makes no sense
  (:unas) value unassigned (e.g., Untitled)
  (:unav) value unavailable, possibly unknown
  (:unkn) known to be unknown (e.g., Anonymous, Inconnue)
  (:none) never had a value, never will
  (:null) explicitly and meaningfully empty
  (:tba) to be assigned or announced later
  (:etal) too numerous to list (et alia).
  (:at) the real value is at the given URL or identifier.

From Special Kernel Standardized Value Codes