By referring to Table C.2, we see Tert-Butyl peroxybenzonate 80% will fall under Concentration 77-100 and therefore should be assigned to UN 3103.
For mixtures, the clue for determination will be for the larger component. When you look for Cumyl hydroperoxide 20% in Table C.2, you will find that since the Concentration is less than 90%, it should be assigned to UN 3109. Thence, Note 18 applies and a Corrosive subrisk label becomes unnecessary.
Now, looking at the blue pages, you will see that the PSN for UN 3103 is Organic peroxide Type C, liquid ★ and the PSN for UN 3109 is Organic peroxide Type F, liquid ★. Type A is too dangerous and cannot be carried, and Type G is not restricted. In this instance, you will consider the larger percentage component, the younger letter (i.e., degree of danger), and you come to a conclusion that the substance must be Dec ared as UN 3103 Organic peroxide Type C, liquid (Tert-Butyl peroxybenzonate 80%, Cumyl hydroperoxide 20%). As you can see from the blue pages, even if you use the same PI 570, the quantity permitted for Type C is less than that permitted for Type F, i.e., the degree of danger is higher.
Still however, when assigning PSN to a mixture, you should not determine by the characteristics of each component, but it is necessary to consider the substances as a mixture since a chemical reaction May take place by mixing them. You will have no other recourse but to refer to the MSDS.