The level, or perceived stagnation therein, of the relationship of characters. The Relationship Ceiling can create the illusion of disinterest in the progression, or just writing, of a relationship when in fact it's due to pure design choices. Figuring out the kind of Relationship Ceiling characters have is often a good measure of how probable a future Relationship Upgrade is. While these ceilings have some basis in reflecting reality, the very noticeable ones are often artificial constructs and the rules governing them tend to hold well over very different scenarios.
Ceilings prevent relationships from developing past the direct relatability of the audience. Regardless of the age or makeup of the cast, relationships portrayed with any depth generally reflect those the audience is familiar with. For these reasons, shows marketed to kids generally avoid romance entirely because of lack of audience interest. For example, in shounen a female co-star is involved in the Serious Business of the story rather than treated as a romantic prospect, generally remaining a friend even if she would want otherwise. Shows for teenagers generally end up with rather shallow romances regardless of the implied deepness, maturity, or importance of it because they emulate (relatable) teenage romances.
Ceilings mark the point where progress would majorly disturb the status quo. This is anathema to long-running stories, so writers will generally avoid them unless they are prepared to modify the story as well. However, prominent but meaningless changes do not break the ceiling. True upgrades are rare and many steps never actually affect the writer's ability to tell a specific story. For example, lovers who marry may break a ceiling while lovers who get engaged do not; the latter only predicts a potential event, is easily reversible and ultimately is just a version of the original. Similarly, lovers who cohabitate may replicate a marriage (or at least marriage-like plotlines) well enough an actual marriage is unnecessary and pointless.
Ceilings are common in subordinate media adaptations of a media property, leaving any major changes for the primary medium for the property to address. For instance, in TV series versions of ongoing feature film series, they sometimes set between films. So, the TV series version can hint at certain things developing, but any major developments are reserved for the feature films to deal with such as Po and Tigress' relationship in the Kung Fu Panda film series, or Hiccup and Astrid in How to Train Your Dragon.
Occasionally, writers have the foresight to bring in a new outside character to break the ceiling. However, if this is the primary reason for the character to exist, they often devolve into a Satellite Character.
Because breaking a Relationship Ceiling is a common Jump the Shark moment, writers may backpedal with a Toilet Seat Divorce or a One More Day situation. To avoid this problem, writers will often save breaking the Relationship Ceiling for the Grand Finale resulting in a Last Minute Hookup to reward viewers with an intimate relationship without having to worry about viewers losing interest after the Relationship Ceiling is broken.