DataCite Metadata Schema v4.1 Standard Values for Unknown Information

The following are a set of standard values that may be used when mandatory property values are not available for various reasons.

(: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)