An instance is a time-based outcome or timeline in the spacetime continuum.
Civilizations that have a 6d existence can travel back and forth in time, but within one instance where all events or outcomes in the past or future are fixed within a 6d framework. Such civilizations are multiversal where each universe of the multiverse is bound to one instance. Causing a time paradox event could break the framework and you would enter another instance.
Once the quantum dimension is accessed (7d), the dimension of infinite instances (8d) is found. This is where there are infinite futures and pasts. Such civilizations are megaversal.
An instance only has meaning in 1r (reality). Outside reality, time is meaningless, and an eon is like a second to civilizations that live there.