Full Idea
General tableau rules for universal modality: a) if σ ¬◊ X then σ.m ¬X b) if σ □ X then σ.m X , where m refers to a world that can be seen (rather than introducing a new world).
Gist of Idea
Universal: a) if σ ¬◊X then σ.m ¬X b) if σ □X then σ.m X [m exists]
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.49
A Reaction
Note that the universal rule of □, usually read as 'necessary', only refers to worlds which can already be seen, whereas possibility (◊) asserts some thing about a new as yet unseen world.