Intercoat is like a colorless paint,thin and offers no protection from the the elements,needs to be topcoated like a base,it's primary use is to holdout or protect your graphics or artwork until the next coat of paint is applied.(like before taping for flames)Topcoat clear is much thicker,it contains high solids and protects against UV rays and the elements.Dupont's clear is pretty thick but,HOK's is thinner, and needs more coats to get the same protection but,that's not always a bad thing.Someone else mentioned that it's more flexible too which is great,and I really like the gloss of HOK's clear.
By the way,you can use topcoat clear in place of intercoat if you had to but,not the other way around.