A modern traditionally-animated television show is a logistical miracle. The process from initial script to finished episode can take months (and sometimes more than a year) to finish and requires teams across the world to complete. To maintain quality and consistency, the showrunners have to make official guides so that the storyboard artists and (typically offshore) animators are on the same page. One mistake in the character models or backgrounds can be costly, so these guides can get oddly specific, everything from ear size to window reflections have rules that need to be followed.
Click a series and see how the cartoon sausage is made: