Full Idea
In 'Tractatus' Wittgenstein is not just thinking of a set of possible worlds (in the modern account), but of a structured manifold within which each 'possible world' is located.
Gist of Idea
Unlike the modern view of a set of worlds, Wittgenstein thinks of a structured manifold of them
Source
report of Ludwig Wittgenstein (Tractatus Logico-Philosophicus [1921]) by Roger M. White - Wittgenstein's 'Tractatus Logico-Philosophicus' 3 'Positions'
Book Reference
White,Roger M.: 'Wittgenstein's 'Tractatus'' [Continuum 2006], p.66
A Reaction
So the modern view has the neutrality of a merely formal system, but LW is thinking of them as the modal structure of reality.