When do the ditch lights flash?

I know certain railroads use flashing ditch lights, even GO Transit where I am uses it. But it seems that there is no consistent rule on when they flash. Sometimes they flash when you blow the horn, sometimes they flash when you ring the bell. Even in some cases they are manually flashed when approaching a railway crossing. Is there any set rule on when the ditch lights are supposed to flash?
 
In some cases the horn rings the bell (depending on engine model) but generally the bell makes them flash. But as stated above not all engines or railroads have flashing ditch lights set-up. CN and CP for example do not and GO, Amtrak does. Not sure about VIA. It's not a requirement either as some of the older units do not have this function.

Thanks

Sean
 
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