Final answer:
Class D portable extinguishers are the ones that typically do not receive a rating because they are used for specialized fires involving combustible metals, which require specific extinguishing agents and techniques.
Step-by-step explanation:
The class of portable extinguishers that typically does not receive a rating is Class D. These extinguishers are designed to put out fires caused by combustible metals and metal alloys such as magnesium, titanium, zirconium, sodium, and potassium. Unlike Class A, B, and C extinguishers, Class D extinguishers do not have a numerical rating since the conditions for metal fires can vary widely and require specialized extinguishing agents. These agents work by smothering the fire and absorbing the heat so the metal doesn't re-ignite.
Class A extinguishers are rated for ordinary combustibles, Class B for flammable liquids, and Class C are rated for electrical fires. Each of these classes is provided with a numerical rating that indicates the effectiveness of the extinguisher in putting out a specific type of fire, unlike Class D extinguishers.