In relation to rear fog lamps:-
Earth predominately harmonizes vehicle lighting regs with UNECE.
The global rear fog lamp function is UNECE 38 (its full photometric criteria, etc), strictly speaking, the rule is mandatory for most road going motor vehicle categories. However, some countries, by localized regulation, "optionalize" the function. An example is Australia, the global governing rule, UNECE 38 is adopted as ADR 52, but is optionalized at ADR 13. It saves some manufacturers a buck.!
In the US, the lamp is optional as its not included in FMVSS-108, but the SAE back in 1983 created its first "recommended practice" for the function,- known as SAE J1319. Now, SAE J1319 had undergone updates in recent time, making the lamp function almost identical the the UNECE criteria, and accepts the UNECE 38 aspects.
The rear fog lamp offers up to 20 times the luminous intensity over a vehicles regular taillights, this equates to many seconds of advanced warning of your vehicle in dense fog, daytime torrential/monsoonal rain storms/blizzards, etc, - resulting in 'white-out' conditions, or in dust-storms, when driving on gravel roads, OR in wildfire/bush fire smoke conditions.
The rear fog light should not be used in a mere light mist, and drivers should switch the function off, the moment visibility improves. German, France stipulate a 50m visibility maximum before the function can be used, GB sets that at 100m, Australia - "under hazardous weather conditions causing reduced visibility".
UNECE working parties on road safety, have no intention of ceasing the function, naturally enough, but will update UNECE 38 to enable new technologies, that will enable to automate the function, for example brightening the rear fog lamp from standard taillight strength up to the functions photometric maximum value, basically small increases in brightness, fluctuating on detected conditions (Championed by Audi at UNECE level).
Attached, is the switch and dash ISO symbol.