@Fungus
Maximum working voltage is maximum voltage that can be applied to instrument in normal conditions on a permanent basis.
It is voltage that will not cause any deterioration of instrument by gradual failure of components inside, and will protect user while measuring, if good practice is followed.
So, If highest voltage you are about to measure is 400V, that is your working voltage. And then you buy instrument rated 600V just to be on the safe side..
CAT II, III, IV is resilience to overvoltage transients, and is related to environment you are working in. If you have 12V off the grid solar system on a ranch in country, and have long cables that go overhead, in stormy weather those can pickup huge transients....
So in that case you would need a CAT IV meter while working voltage of 24V would be sufficient, if such thing existed....
By same token, if you are repairing 800V tube amplifiers in your well protected lab in a city, you might need voltmeter with 1000V working voltage, and CAT II, or even CAT I might be fine for you.
Simple as that. Well, not really, but that is basic logic. Trick is that most of the time it is hard to know in what environment you are/will be working, so people prefer to err on the safe side.
Regards,
Sinisa