This is my understanding, so CMIIW
Short is: When you are ontologically more real than a certain reality.
For example, the A verse has 3 Main/Primary Universes, these universes is the main universes where all things within the verse is happening, like the story, feats, scaling, etcetera. So we consider these universes to be the ultimate reality, but there is a being who exists in a different reality and considers that everything that happens in those universes is just fiction. They transcend material reality by nature, not by scale, and cannot be reached by any amount of physical or dimensional extension because their superiority is ontological, not numerical.
So, even an infinite, multiversal, or higher-dimensional universe would still be “fiction-like” in this/their sense.
So, put it simply like whenever a character sees a certain reality/universe(s) merely as a fiction ontologically.