From Creatures Wikia
The term family is the top level of the Classification system. Agents with a family of 1 are invisible to creatures. Agents with a family of 2 or 3 are usually visible to creatures. What specific type of object an agent is classified as depends on its genus.
Agents with family 4 are special -- they are creatures. To enumerate over every creature in the world, try this:
enum 4 0 0 [code] next