In addition to Tag 666 and Tag 667, Strife supports two additional special tags which have special effects on any map when all monsters of a given type die and there is at least one player alive to claim the victory.
Tag 999 is activated when Spectre A is defeated. All sectors tagged 999 will lower their floors to the lowest surrounding floor. Spectre A is the Programmer's spectre, which is normally found in MAP25: Catacombs: Ruined Temple.
Tag 999 is additionally used to mark shopkeeper doors. When the player sets off the alarm in the presence of any shopkeeper, all sectors tagged 999 will close as blazing doors. The doors will reopen when the alarm deactivates.
In the United Kingdom, 999 is the dialling code for the emergency services including the police — which fits the purpose of this tag. However, as Rogue were a Texan company, this is almost certainly a coincidence.