In the proposition ' Socrates is an animal ' the term ' Socrates ' signifies, as a conventional sign, a particular individual thing (also outside any proposition it signifies, as a conventional sign, that same individual). But in this particular proposition the meaning of the term ' Socrates ' is restricted : Instead of referring to the whole supposit it refers only to the nature that resides in it as its specific Essence. This nature is humanity.
The intrinsic meaning (signification) of the term ' animal ' extensively refers to all individual animals, and intensively refers to the nature animality. But the term ' Socrates ', referring in our proposition to the nature humanity, restricts the meaning of the predicate ' animal ', which intensively was animality, to rational animality, which is equivalent to humanity.
So also here we can say : Although the terms ' Socrates ' and ' animal ' signify different entities, used in the proposition ' Socrates is an animal ' they designate the same, and can therefore be identified with each other.