The Cover rule is a rule used to classify interactions between series that happen on the cover of anthological works, and similar illustrations:
Elements from different series appearing together in an illustration create an in-universe link only if the illustration features narrative elements, otherwise it doesn't count as a fictional link.
This is because an illustration can only be considered a fictional work if it tells a "story", otherwise it's just a promotional image and it's unnotable to this wiki. In general, when characters are just seen posing for the "camera", that doesn't count as a link, when they're seen in a situation that requires background, like living an adventure together, that counts as a link.
Some cases might be arguable, such as characters performing a generic activity, like playing sports or driving cars: that scene tells an event, but it doesn't require any background and could still be considered just posing. Such cases are open for interpretation and should be noted as arguable in the detailed link page, letting readers come to their own conclusions.
If we need to make a choice on arguable cases, this wiki's directive is that if the scene shows a situation that makes sense, then it can be considered a plausible situation and it counts as happening in a fictional universe creating a link, if it shows a theoretical situation that doesn't make sense then it's still just considered a promotional image.
Of course, if elements are together on the cover of an anthology work, the rule is only worth discussing if the elements don't also meet within the work itself. If they do then it's a legitimate crossover work, and the interactions on the cover doesn't matter.
This rule can be extended beyond illustrations: commercials or television channel bumpers might show characters together in a small video. These are considered links only if there's some narrative element to the video beyond just posing for the camera.
Issue 15: Namor is shown tied to a tank and Human Torch is flying to his rescue. This has a clear narrative component, therefore counts as a link.
Issue 22: Both heroes appear on the cover, but the image shows two separate situations. This doesn't count as a link since there's no interaction between the characters.
Issue 84: The four featured heroes appear together on the cover, but they're just posing, therefore this doesn't count as a link.
Issue 88: the four characters are shown relaxing together, therefore there's a narrative component and it counts as a link. Human Torch breaking the fourth wall doesn't affect this since the line is beside the interaction.
World's Finest Comics is a comic book series by DC Comics featuring stories of Superman, Batman and other heroes. While starting with issue 71 the series included crossover stories, the heroes were sometimes shown meeting on the cover in previous issues:
Issue 1 (World's Best Comics): Superman, Batman and Robin appear together on the cover, but they're just posing, therefore this doesn't count as a link.
Issue 3: The three heroes are seen playing baseball. This is an arguable case: while this is an activity, the scene has little narrative value. Since it makes theoretical sense, we count it as a link, but its arguable nature needs to be noted.
Issue 7: the three heroes are seen sitting on a warship's cannons. This can be considered just posing, since it doesn't really make sense for them to be sitting there.
Issue 51: the three heroes are saving a woman from a fire. This is clearly an adventure requiring its own backstory and therefore definitely counts as a link.
Issue 68: the three heroes are running away from a skunk. While this scene is comical, it still features a narrative element and can be considered a link.
Star Comics Magazine is a comic book series by Marvel aimed at children, including various licensed stories such as Heathcliff. While characters usually don't meet in the stories themselves, they sometimes do on the cover:
Issue 1: Heathcliff is shaking a magic wand and other characters are jumping out of the comic's pages (Applying the Toy Story rule, the characters brought to life through magic count as real incarnations). Heathcliff is just performing an activity, so it's an arguable case. Since the scene doesn't seem to be plausible, we don't count it as a link.
Issue 2: Heathcliff is blowing bubbles, and images of the characters appear in them. This doesn't make sense, with the characters being shown just for promotional purposes without an actual interaction between them, therefore it doesn't count as a link.
Issue 5: Heathcliff flies on a rocket with images from the various stories appearing in smoke clouds, and then the characters are shown hitchhiking on a small planet. While the clouds clearly don't count as interactions, the hitchhiking part has narrative value and counts as a link, although it doesn't make much sense.
Issue 8: the characters are posing for a photograph. Despite their activity literally being that of posing, the photographer (a Madball) is providing a narrative element through his comment, making this count as a link.
Spider-Man is seen swinging with various characters from the comic. They aren't doing much beside the basic activity of swinging, so this counts as an arguable link.