see also: "fourth wall" on Wikipedia.
The fourth wall breaking or breaking the fourth wall is the act by a character in a fictional work of speaking directly to the audience, acknowledging to be a fictional character. In the context of crossovers and cross-series references, the following rule is applied to fourth wall breaking:
- If a fictional work features a reference to another series during a fourth wall breaking, that's NOT considered a fictional link.
This means characters starting to talk directly to the audience and as they do so, also referencing another series. That's not a fictional link because links are meant to indicate a relation between fictional universes: if an element from a series appears or is referenced in another series, means that that series' universe is connected to the other one, for example revealing that events from both series happened in the same universe, or that a series is fictional within the other series. During the fourth wall breaking, though, the creators of the work talk directly to the audience, even if they do so through a fictional character, so that counts as a message from a real person to another real person, it happens in our universe, not in the character's universe, so it doesn't reveal anything about the fictional universe.
Note that we're referring to references happening exclusively during a fourth wall breaking, that is a character talking about a series while breaking the fourth wall; the other way around, a character breaking the fourth wall while talking about another series, still counts as a link, because the character was talking about that series also outside the fourth wall breaking.
This is usually done for commercial purposes, with a fictional character advertising a work from another series, directly speaking to the audience. Another case is that of a character reviewing a fictional work. Note that commercials and reviews are generally not considered fictional works as a whole, but there might be exceptions in the form of commercials or reviews that also have a narrative component. (see AVGN rule and Commercial rule)