[Clause 9.32.3.11.(6)(a)] To limit the probability of melting or burning of the duct on exposure to heat from a fire or to corrosion, which could lead to the perforation of the duct, which could lead to the growth and spread of fire, which could lead to harm to persons.
[Clauses 9.32.3.11.(6)(b) and 9.32.3.11.(6)(c)] To limit the probability of the entry and accumulation of grease and embedded dust in the exhaust fans or ducts.
This is to limit the probability that, in the event of a cooktop appliance fire, the ductwork will contribute to the spread of flame and smoke, which could lead to harm to persons.