Sorry for the very belated reply, but I was struggling to find the words to describe exactly what I meant. Luckily, Eliezer has already done most of it for me in his latest post.

Thing A exists with respect to Thing B iff Thing A and Thing B are both part of the same causal network. So ArisKatsaris was half-right, but things outside our past and future light cones can be said to exist with respect to us if they have a causal relationship with anything that is inside our past and future light cones.

Wrongbot's idea is probably that something "exists" only in relation to something else. So things beyond the Earth's past and future light-cones don't actually "exist" for us, though they exist for themselves, and similarly things in a mathematical model of a different universe exist in relation to each other, but not in relation to us...