In NGK terms, the higher the number, the colder running the plug is.
In (VERY) simple terms, the more power your engine develops, the more heat it generates in the process....and the more heat the spark plug needs to dissipate. That means a colder running plug.
All things being equal, ie the right grade selected, all plugs in all engines run in pretty much the same operating temperature envelope.
It is always safe to put a colder running plug than recommended in an engine - the only risk is that it will foul. It is never safe to put a hotter running plug than recommended in an engine - the big risk is it will overheat and cause pre-ignition.
That's a BIG adavantage of precious metal plugs - they run very cool but the electrode material makes them less sensitive to cold-fouling products.
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