  We give a classification of substructures (= closed subbifunctors) of a given
skeletally small extriangulated category by using the category of defects, in a
similar way to the author's classification of exact structures of a given
additive category. More precisely, for an extriangulated category, possible
substructures are in bijection with Serre subcategories of an abelian category
consisting of defects of conflations. As a byproduct, we prove that for a given
skeletally small additive category, the poset of exact structures on it is
isomorphic to the poset of Serre subcategories of some abelian category.
