Have you tried the manufacturer? My crystal ball is out to be repolished, so I don't know what brand of generator you have, which might be important if you want a correct answer.

Call the manufacturer.

Been doing a lot of gazing and scrying again have we?

Your comment made me look at he OPs thread history - didn't realise he was a student. He's got a lot better at hiding it over the past few months hasn't he.

Without manufacturer and model number data, my guess;

Idle speed setting keeps the unit running at idle speed most certainly under a no-load condition.

OR

Idle speed setting will allow the RPM's to fluctuate based on load demand (unlikely with an 800kw unit).

Rated speed setting runs the unit at the specified RPM to produce power (voltage) under load.

Your managers obviously don't trust you.

Otherwise they would willingly give you this information if it was required to you.

Please carry on in you're day to day job.

IDLE speed is normally provided for situations where you want to run the genset but there is no possibility of a load going on. Generally around 650-800RPM. It depends on the genset usage whether IDLE speed is useful to you or not.

GA. That's the answer I was going to give.

The idle setting also allows the donk to be started and rotating unloaded in a "warm standby" state ready to spool up to speed quickly prior to accepting a load.

Idle speed is also a lot quieter for when you need to listen for and identify the sources of odd noises.

Idle speed is fast enough to charge the cranking batteries too if that's all you need to do.

A little imagination and some experience could come up with other situations where idling would be advantageous.

This may be used for changing the meter readings between rated & idle.

Idle speed is for ensuring the circulation of the lubrication & the cooling medium to coold down the hot parts of the engine. Typically, for a 1500RPM rated speed engine, the idle speed would be around 1000RPM.