The Lenny Leggo Canon is essentially the official "Universe" or timeline, that The Lenny Leggo Show takes place in.
Here is a list of what is considered canon and what is not canon.
With the exception of Season S (see section below), all episodes of The Lenny Leggo Show are canon.
Furthermore, all shorts, which includes but is not limited to bonus shorts and season trailers, are also canon.
Both Unmemorable Jonah and Contributions, for example, are canon: they take place in the same continuity as the show. The former is mentioned to take place on Memorial Day, meaning it actually takes place in between Season 1 Episode 1 (Unhappy Birthday Martin Anderson) and Season 1 Episode 2 (Fireworks). The latter takes place during an unspecified point in time prior to Unhappy Birthday.
Unless otherwise specified, a short released during a given season's run always takes place during that particular season. For example, Voting Night, which was produced and released during Season 1's original run, takes place during that same season (more specifically, between National Jonah Flenderson Day and Thanksgiving Kerfuffle, the former taking place on or before October 31st 2016 and the latter taking place the day before and on Thanksgiving Day). Furthermore, shorts that are released between seasons also take place between them as well. For example, Fewhales, the Season 2 trailer, is canon and takes place before the Season 2 premiere.
Deleted scenes are generally canon, however, for a deleted scene to be canon, it must not contradict any events or story elements in the final cut of an episode: if it does, it is automatically non-canon.
The Studio ContinuityEdit
The Studio Continuity is a separate continuity that features Aaron Cosby (as himself), who produces the show while the characters, in animated form, are presumed to live with him at the studio.
Season S is a "special" season of episodes that do not fit in with the rest of the primary timeline. Such episodes may include parodies, episodes set in time eras other than the present, etc.
Deleted Scenes (that contradict an episode's final cut)Edit
The general rule is that deleted scenes are canon if they do not contradict any events that occur in the final cut of an episode. Such deleted scenes that create contradictions are ruled out of the canon. Such deleted scenes are usually scenes that are replaced, rather than simply removed (the latter case is such with most deleted scenes).