Full Idea
A 'necessary being' is one that exists in every possible world; and only some objects - numbers, properties, pure sets, propositions, states of affairs, God - have this distinction.
Gist of Idea
Necessary beings (numbers, properties, sets, propositions, states of affairs, God) exist in all possible worlds
Source
Alvin Plantinga (Actualism and Possible Worlds [1976], 2)
Book Reference
Plantinga,Alvin: 'Essays in the Metaphysics of Modality' [OUP 2003], p.110
A Reaction
This a very odd list, though it is fairly orthodox among philosophers trained in modern modal logic. At the very least it looks rather parochial to me.