Fundamental Assumption of The Expanded Multiverse

The Fundamental Assumption that serves as the basis and starting point for Expanded Multiverse Wiki is what follows:

If an element from a work of fiction is acknowledged to be real to another work of fiction, then the two works are either in the same universe or they are parallel universes in the same multiverse.

This is an axiom that is taken as true, and is therefore not arguable nor provable. It represents a philosophy that not everybody must subscribe to, but if one chooses to subscribe to it, then this Wiki's purpose is to explore, classify, and describe all its consequences, as well as to discover how large can a multiverse defined this way get.

Premises
While taken as true by default, the Fundamental Assumption mentions a few keywords that need explanation:
 * Work of fiction: a work of fiction is any narrative work that tells events not based on history or fact. This includes works from any possible narrative medium, but excludes any work that is not strictly imaginary in nature, such as manuals, biographies, or essays.
 * Element: an "element from a work of fiction" can be any imaginary thing belonging to said work of fiction, including but not limited to characters, items, places, organizations, and even fictional works of fiction. Note that for the element to be classified as "from" a work of fiction it should belong to it, meaning that it either originated in said work or is a central part of it. Most importantly, this means that acknowledgments of non-fictional elements such as real people or real places, do not count.
 * Acknowledged to be real: an acknowledgement can range from characters and events of the referenced work being prominent in the referencing work, to minor background cameos, to mere mentions. For the element to be considered "real", the referencing work must specifically imply that the element is not imaginary within its fictional universe, and it must be the actual element rather than an imitation. For more details see the article on Fictional links.
 * Universe: the term "universe" in the context of works of fiction has two main possible meanings: it can either indicate the "fictional universe" serving as the setting for the story, or the "narrative universe", also known as "canon", identifying the narrative thread that is told throughout one or more stories. The difference is mostly relevant for stories that implement a plot about alternate universes: characters are sometimes explicitly said to leave their home universe to explore a new one, sometimes meeting their alternate counterpart, but being a singular consistent story it's still clearly one single narrative universe. This Wiki classifies both cases, generally identifying the fictional universe as Universe and the narrative universe as Continuity; see the respective articles for more details.
 * Parallel universes: this means that the two universes are considered linked in an ideal way. In this context the term "parallel" doesn't necessarily mean that the two universes "never meet", as implied by the geometrical definition of the word (in fact it sometimes happens that universes "collide" in works of fiction), but rather that there's a "parallelism" between the two universes, that there's something in common in things like their structure, their events, or their inhabitants.
 * Multiverse: a multiverse is a set of parallel universes. The Fundamental Assumption doesn't specify if the parallelism is transitive for universes, so while it's sure that two linked universes share the same multiverse, it can't be said for sure to what extent can multiple parallel universes form a single multiverse. For more details see the article on Multiverses.

Consequences
The Fundamental Assumption has a set of direct an indirect implications:
 * World building: the most basic application of the assumption comes into play in works with a consistent narrative. When a story is written as a sequel to a previous story the two can clearly be assumed to exist in the same universe. By transitive relation multiple works can start sharing the same universe and a larger consistent fictional universe can be created.
 * Shared universe: an acknowledgement might reveal that two otherwise unrelated works of fiction actually happened in the same universe all along. This might have been directly meant by the authors or it might be an unforeseen implication of the acknowledgement.
 * Series multiverse: when it comes to works of fiction being adapted into new works, such as remakes, transpositions to different media, or in general by reusing the same elements (such as main characters) to write a new story, the Fundamental Assumption dictates that they are parallel universes in the same multiverse. This means for example that the original series of Tomb Raider games, its comic adaptations, the first series of movies, the rebooted series of games, and the 2018 movie are all necessarily parallel universes in the same multiverse. A multiverse defined this way is called a Series multiverse; see the relative article for more details.
 * Expanded multiverse: defining subsequent pairs of parallel universes allows us to identify a set of universes that can all be connected this way. Such a set is called an Expanded Multiverse; see the relative article for more details.

Same universe or parallel universes?
Whenever an acknowledgment happens between two works of fiction, the Fundamental Assumption dictates that they are either the same universe or two parallel universes, but when is it one and when is it the other? An absolute answer can't always be given, but when both possibilities are plausible this Wiki applies Same-universe priority; see the relative article for more details. Even when accepting this additional assumption though, many cases remain uncertain and open for interpretation: if a certain work of fiction A acknowledges both work of fiction B and work of fiction C and it can happen in the same universe as either work without discrepancy, there might still be some discrepancy between works B and C preventing all three works to share the same universe. In certain cases the solution might be that the universes of B and C are separate and work A happens in both; in certain cases priority is given to historical consistency (meaning that if the acknowledgement of work B happened first it revealed that work A and B share the same universe, so when the acknowledgement of work C happened that's considered a parallel universe); in other cases priority might be given to the stronger acknowledgement (for example if acknowledgement B is only a background cameo, while acknowledgement C is a major element tying the plot of the two works together, then A and C are considered to share the same universe while B is considered a parallel universe).

In any case it should be noted that regardless of the nature and subsequent implications of the acknowledgement, the Fundamental Assumption makes sure that the two works of fiction certainly share the same Expanded Multiverse.