Hello there,
Seasoned engineers and electronic workers know that when something is described
as "current operated" or "current controlled" that means that the MAIN parameter
is current, not voltage. They also know that this doesnt usually mean that there
will be no voltage present across the device, just that the main parameter to
pay the most attention to is the current rather than the voltage.
In other words, you can work well with a 'current controlled' device by setting
the current at a particular level and paying little or no attention to voltage,
rather than setting the voltage to some preset value and forgetting about the
current level.
In the case of an LED, you do better to set the current to 20ma and not worry too
much about the voltage (voltage is of secondary importance) than to try to set
the voltage. Of course voltage has to be considered to some degree, because
of course you have to have enough voltage to feed the LED too, but it will
be less important. For example, we may set the current to 20ma and if the
voltage is 2.0v or 2.1v or 2.2v or even 2.4v we dont really care that much, but
we would never be able to set the voltage to 2.2v (say) and forget about it
because we wouldnt be sure the current is right, and the current being right
is the most important aspect of running the LED: too little and it is too dim,
too much and it burns up.
If this was a bulb however, we would be more concerned about what the
voltage is because that affects the operation the most.
So in the end it's mostly a matter of priority: current or voltage, depending
on the device. We call a device who's operation depends mostly on current
a 'current controlled' device, and a device who's operation depends mostly
on voltage a 'voltage operated' device. There are variants too, like before
'current operated' or even 'current mode', to name a couple.