With regard to Fan Sensors, there are two very common methods for detecting whether the fan is doing what it's supposed to or not.
Locked Rotor- Detects when the fan is working, or when it has stopped. (on or off voltage signal)
Tachometer- Detects the speed of the fan, to determine whether it is performing optimally. (pulsed voltage signals)
Q- What is the difference between using these two types of fan sensors in 'some' design?
I don't know what this 'design' is, but I'm asking for the practical differences if I picked either one.
I understand that both setups use different voltage signals, and require different circuitry to operate. That is beyond what I think would be the practical scope of this question.