DC or pulsed DC (PWM).
It's industrial standard to have function outputs as open collector transistors, so you have a common plus (orange wire or blue wire) with function outputs switching against minus.

Some outputs are logic level where "1" is +5V and "0" is 0V (maximum load typically 2 mA). These outputs need external amplification. You find those on the "mSD Spezial" and with ESU mfx decoders sold by Märklin.

It depends how you wire things. There is no difference between light outputs and smoke generator outputs.
Smoke generators are normally connected between function output (minus) and outer rail/loco frame (sometimes minus, sometimes plus), so the smoke generator will get power about 75% of the time with MM protocol. Some sort of PWM, based on the shape of the track signal.
Lights are normally connected between decoder plus and function output (minus) and get the "full voltage" all the time.

The track voltage can be anywhere between 16 V and 22 V. Inside the decoder there is a bridge rectifier and a transistor, so the voltage at the function output will be about 2 V below track voltage.

To calculate resistors for LEDs, I'd reckon with 22 V to be on the safe side. Most LEDs will be too bright anyway, so with these old decoders some testing with different resistors can't harm.

29640 is a 2012 new item - you probably mean a different number.

To resurrect this old horse 4 years later....

I'm trying to spec the micro-relays I would need to use a function output from the factory installed mfx decoders on modern locomotives as a driver. The plan is to use an existing function output on the factory installed mfx decoders to drive a micro-relay to switch power on/off to a conducting coupling to flow out to the chain of 3-6 cars' LED lighting.

So I'm trying to spec the coil voltage of said relays. Do the Märklin factory relays generate logic level voltages (eg 5vDC - or can I switch their open collector nature to a 5vDC source on the decoder) - or do they generate 19vDC...

I probably should just open up one of my locos and measure, but am trying to save myself that exercise (as they are all presently boxed up and stored as I prepare the room for layout construction).

Or do I just spec a 5vDC coil voltage relay, and then install the appropriate resistor to ensure the coil voltage is 5v based on relay's coil resistance?

I guess my next question is how can one tell. Any technical docs on this?

2mA isn't a lot to work with