I guess I have to back up a bit from my post 6, After looking at some of the proxy's we use at work most of them are in-fact a sensor even if they have a NO or NC output, I still stand that a sensor is a variable output device and a switch is a off or on device, but now that technology has reduced the size of electronic components so that a proxy sensor can include the oscillator, amplifier, and relay or output switching transistor all in the same package many proxy sensors now have the controller and output all in the same package, not many years ago this was not true as a proxy sensor had to have the controller as a separate unit.
But how the sensor operates is still the same, the heart of the sensor provides a variable output to an amplifier that is used to switch a switching transistor which can drive a relay for a dry contact output, or it doesn't have the relay but still has the switching transistor which can switch a load depending on NPN or PNP that can be used to control a PLC input signal either pull up or pull down input.
Or leave out the switching transistor and have a variable output such as but not limited to a 4-20ma or 0-10v PID output that a controller or PLC can use to make and change set points to adjust the the point of change you want or need, these are called Analog & Linear outputs.
But the heart of a proxy sensor is the fact that it does provide a variable voltage or current to a amplifier that can be used for your control, the fact that a certain models might also have the controller with a relay on-board with adjustments to change the point it switches the relay or changes the sensitivity does not change this and is why we see these still being called sensors and not switch's.
When I hear proximity switch the first thing I think of is the old magnetic reed switch's that had a small magnet on the read switch that came in just a few types NO and NC or both, these were nothing more then a switch, and was very common type used for alarm circuits on doors or windows that can slide to open, there are also some that use electronics to provide the same results but are basically the same, they have no variable output they are either open or closed NO or NC.
As to post 10 (jusme123)
No most proxy sensors have adjustment depending on its design and model can have many adjustments and some can even have time delays to prevent false on or offs, these are all in one sensors with the controller built in and package and a single mountable unit.
Also any switch that has to make physical contact with the item it is used for is nothing but a limit switch, while a limit switch can be used as a proxy switch for position it is not defined as a proxy switch as it does make contact which a true proxy switch as the name implies only has to be in proximity of the item that is being detected, simply a proxy switch or sensor is a limit switch that doesn't make contact with the item it is monitoring.
There are many types of proxy sensors and the use for them is endless, from the wire embedded in the road to detect cars that make the signal light change or prevent an automatic gate closer from closing on a car, to an IR laser reflected off a target that determines the distance to the target for laser guided missiles, you have four basic types of proximity sensors: infrared, acoustic, capacitive, and inductive but there are others including radio wave systems.