The very first example given of when to use "c'est" is:
"C'est une jolie robe."
This clearly refers to a specific dress.
But then we are told that the answer to "Tu aimes mon pull?" should be "Oui, il est très beau."
Why is the rule for a specifc dress different than the rule for a specific sweater? Is it that "il/elle est" should be used when answering a question about a specific thing but that "c'est" can be used otherwise?