Hello,
Since my name is Phil I will probably get "filled in" on the subject as well as you will. I do not claim to be all knowledgeable about 240's.

With that said I will give you what I know,
The Head light relays started being used on the 1985 models and forward.
The four bulb front lights ended in 1984 on North American models and the plastic lens types were introduced.
This relay only is for are the cars that have their headlights turn off when you turn the ignition key off.
It will only light up the headlights with the key on and the headlight switch on.
This relay is located to the right of the instrument cluster in the dash.

A low/high beam dipper switch is another relay located on the driver side fender well out under the hood. It is operated by the directional turn signal stalk on the steering column.
It is what is sometimes called a "I'm wanting or going to be passing your car" warning light or a "please dim your headlights" to on coming traffic in most states.

Basically, it's also an attention getter when on held on. It will operate with the headlights switch off and the ignition switch off. It's nice to blink in a parking lot to say "Honey, I'm over here."
Sometimes, if and-when, the headlight switch or relay total fails, the dipper relay will hold your high beam headlights on so you can drive.
Think of it as a redundant "NASA type" back up system only with human intervention, otherwise, you don't get home! Any car can help you into an APOLLO moon mission.


As far as the bulb out or Bulb Failure Sensor, it is independent of any relay circuit operation, as wired from the factory.
If one bulb draws more or less than it's "brother- like" bulb it will trigger the dash light.
IT does the walking around the car for you but it's still a RELAY that turns on a warning light.
I have read, it can do more a it can turn off lights that are fine.

What are you up too?
On what type of 240 are you working on or modifying?

Phil