This could be another example of bad usability.
An optical attenuator is - without doubt - a useful device in lab works. A variable one is suitable for many practical tasks. One of the variable optical attenuator from a well-known vendor has a front panel like this (I know, this sketch is lame):
The seven-segment display shows current attenuation level in dB. It can be changed using the three buttons next to the display (under the label VERNIER).
The INPUT and OUTPUT ports are very clear, they are where you plug the fiber connectors. The light coming out from the output port has less power (as it is attenuated) compared to when it enters the input port.
The POWER switch (it's toggled switch BTW) has a red LED. The meaning of this LED is also quite clear. If it is on, then the device is powered on (which just makes sense).
It is the DISABLE switch that is quite of a question. This switch has also a red LED attached to it. However the meaning of this LED can't be deduced easily, or you may need to consult the handbook to confirm your understanding. For example, if we take it that when the LED on the POWER switch is on than the unit is powered, does that also mean that if the LED on the DISABLE switch is on than the unit is disabled? Or is the other way around, an active indicator (like the LED) means that the unit is actually enabled?
And what does disabled actually mean? Does it mean that no light is coming out from the OUTPUT port, i.e output is fully blocked? Or does it mean no attenuation takes place (i.e. the output power is the same as the input power?
This looks simple, but consider that the light might end up in a sensitive (and possibly also expensive) photo detector. Yes, people shall read the handbook first, but such simple matter should be straightforward anyway.