Explanation:
That you cannot go faster than 40 km/h when there is fog: Correct
The main sign: Maximum speed limit 40 km/h. The complimentary plate with the car + rain/splash lines applies only when the road is wet (rain).
P-33 Reduced visibility is a warning triangle with a car and three diagonal black lines like fog or rain, this sign warns of general reduced visibility (fog, rain, snow, smoke, dust, etc.). The panel is rectangular (complementary panel) placed under a circular speed limit sign, which makes it different in meaning. It is used to lower speeds specifically on wet pavement, which is by rain and not wet with fog