Full Idea
Believers in S5 as a correct system of propositional reasoning about what might have been must claim that it is an essential property of any way things might have been that things might have been that way.
Gist of Idea
S5 believers say that-things-might-have-been-that-way is essential to ways things might have been
Source
Nathan Salmon (The Logic of What Might Have Been [1989], V)
Book Reference
Salmon,Nathan: 'Metaphysics, Mathematics and Meaning' [OUP 2005], p.145
A Reaction
Salmon is working in a view where you are probably safe to substitute 'necessary' for 'essential' without loss of meaning.