Summary
Modal Realism is an idea that was proposed by David Lewis, in which proposes that all logically possible worlds in tandem with the actual world we currently exist in, with these possible worlds being derived from "modal statements" that are, in themselves, possibilities that tell us something about what could be or must be true. Within this system of reality, all possible worlds are akin to objects with everything in the object being connected in some way but entirely separated from all other objects. This necessitates that possible worlds are just “different ways things could have been”, meaning that they must be similar to the world we live in but with variations
Extended Modal Realism
An extension of the concept of Modal Realism is the proposition of Extended Modal Realism, which takes the ideas already present within Modal Realism and extends it to ontological commitment that also implicate the existence of impossible worlds, which are derived from "modal statements" that lie within contradiction and illogical premises, such as "1+1 = 3" or "what if fire was so cold it was hot". Within Extended Modal Realism's interpretation of reality, modality as a whole is a dimension similar to space and time, in the sense that it also pertains to possible and impossible worlds. This means that everything that can be considered real, whether rooted in possibility or impossibility, exists in some form
How This Treated On-Site
When it comes to both Modal Realism and Extended Modal Realism, their natures make it hard to prove visually within a verse and thus in order for one to argue that it's applicable to a cosmology, it has to be either explicitly namedropped by a character or the series has to have some evidence that it links to either concept (whether that's the creator themselves subscribing to one of these ideas or some other key evidence that implicates it's existence). Anything outside of this would not allow Modal Realism or it's counterpart, Extended Modal Realism, to be applied to a fictional verse given the broad nature of both concepts that would make it impossible to prove otherwise
In the instance that this requirement is fulfilled, where both concepts would scale is dependent on how it's portrayed within the series. Modal Realism, by necessity of arguing that everything that can be logically possible is a possible world, would be at least Low 1-A by virtue of pertaining to real coordinate space and thus an uncountably infinite amount of higher dimensions. It can be argued as higher but this goes based off of how it's described and portrayed within the fictional universe itself
As far as Extended Modal Realism goes, the same principal applies, in the sense that it ultimately depends on how it's portrayed within the work of fiction but as a absolute lowball, it's 1-A by necessity of encompassing real coordinate space. When taken to it's most logical extreme and with the assumption that all implications surrounding it apply to the fictional universe, it would easily scale to 1-A Ω but this is the absolute best case scenario