For example, the predicate ' rational ' is not essential to animal qua animal. So taken as such we cannot assess the status of ' rational '. Only connected with a subject this is possible, for example with a definiendum : For only because there is a definiendum, ' rational ' is an essential predicate : Man [definiendum] is (a) rational (animal) [rational is a part of the definiens].
In ' an animal is rational ' (in the sense of : a given animal is rational) rational is not a (part of the) definiens (i.e. it does not occur in the definition of ' animal ' ). Therefore here animal is not a definiendum, from which it in turn follows that here rational is an accident (in the sense of : not essential). Indeed the sentence means : This animal here turns out to be rational.