Explanation of "Extraneous/Filler" Episodes
An extraneous episode, also known as a "filler" episode, is one in which nothing important or plot relevant occurs. Basically it is an episode that completely maintains the status quo: nothing changes or is revealed that is important to the main plot or any subplots (excluding subplots unique to filler episodes). Sometimes there are minor or subtle hints at plot relevent subject matter, but never anything substantial enough to merit a 'Character Development' tag. However, a filler episode may share a plot tag with, for example, a 'Detective Boys' episode or a 'Ran & Shinichi' episode, but that merely means those themes were touched on, not that any new information or character development occured.