Glossary for detailled explanation - take your pick:
The voltage drop is measured above the switched sensor with maximum electrical load and lowest environmental temperature that is allowed. For positive-switching DC sensors the voltage is measured between the signal output and plus of the supply voltage and for AC sensors the voltage drop is measured above the sensor.
The voltage range indicates the value the supply voltage should not exceed or fall below to guarantee a secure operation of the sensor. DC proximity switches (sensors) have to be used with an low voltage protector.