Object(n).expression lets you pick the nth instance of am object in an expression.
construct.net/en/make-games/manuals/construct-3/project-primitives/events/expression
See the section on object expressions.
Iirc, this is special in that it can reference the values of an object that isn't currently picked by event conditions.
There is nothing special about object.anything, as long as object.anything is any expression that returns a number that represents the object instance (n, or iid) you're trying to get at.