Definition
A duty rating is the manufacturer's definition of the maximum output an engine may produce and the operating pattern it may be used in, such as the hours per year and the proportion of time at full throttle it is designed to tolerate.
The same engine block is commonly sold at several ratings. A high rating assumes light, intermittent use — a planing boat run hard occasionally. A continuous rating assumes long hours at high load, and is set lower to survive it.
Why it decides more than the horsepower figure
Running an engine outside the pattern its rating assumes is the fastest way to shorten its life. A light-duty rating installed in a vessel that works long hours at high load will not last, however many hours the meter shows today.
This is the single most useful thing to establish about a commercially derived engine when buying a boat — see how to assess a used marine engine and the Caterpillar and Cummins brand pages.
Where you meet it
In manufacturer data sheets, on the engine plate, and in survey reports. Ask for the rating as well as the model when comparing two apparently identical engines.