https://www.newnets.ru/knowledge/19942/

In Power over Ethernet systems, the power sourcing equipment (PSE) senses the Ethernet connection to detect if the powered device (PD) is plugged into the cable termination by applying a small voltage (2.7 V to 10 V) on the Ethernet cable and measures the equivalent resistance in at least two consecutive steps. During this phase, the PD must present a resistance between 23.75 kW and 26.25 kΩ.

Designers can set the current by changing the value of the external resistor according to the following table:

Table 7: Value of the external classification resistor for the different PD classes of power