The concept of categories not overlapping in content.
Categories must be labeled and defined significantly different from one another to ensure any one item has only a single home in the taxonomy. A break in mutual exclusivity may indicate that sibling categories were created along multiple points of differentiation. Some or all of the differences may by better handled using attributes instead of categories in the taxonomy. By removing the overlap among categories, product managers and customers can easily find what they are searching for throughout the hierarchy.
Parent Category, Sibling Category