When it comes to crossovers between characters from different comic book companies, I.e. Marvel and DC, there are many. How the crossover is accomplished can be different per story: Sometimes heroes from one universe jump across realities to another, and other times, heroes of both continuities are presented as occupying the same plain of reality. There have been times when these crossovers have been canon to one or both continuums. DC vs. Marvel and JLA/Avengers were both crossovers of the first type, and both counted. Some crossovers of the second kind seem to count too, as is the case with Batman/Punisher, the events of which were referenced in an issue of the mainstream Batman series. But Batman and Punisher both inhabit totally different universes, right? So how do they meet the way they do here, with no inter-dimensional travel? The best solution, IMO, is that these events occur in a hybrid reality temporarily formed when two universes merge; they soon separate back into their proper forms, though some memories of them persist for a while in the inhabitants afterwards. But it's when we get to JLA/Avengers that problems crop up. This crossover is of the first kind - inter-dimensional travel - and is canon, as it had repurcussions in mainstream DC, but it posits that any merging of two distinct universes results in widespread cataclysm and destruction....none of which was evident during the earlier mergers which must have occurred. So, from an in-story perspective, how to account for this?