The POE standard provides detailed regulations on the electrical parameters of PSE equipment and PD equipment. Equipment that meets the POE standard must meet these electrical parameters, otherwise incompatibility between PSE and PD equipment may occur.
The IEEE 802.3bt standard was finalized in November 2018, but has not yet been officially released. The main differences between IEEE 802.3bt and IEEE 802.3af/at are as follows:

Firstly, IEEE 802.3bt triples the maximum power of power supply equipment (PSE) from 30 W to 90 W, and increases the power level of consumer equipment (PD) to 71.3 W.
Secondly, IEEE 802.3bt has increased the number of levels supported from 4 to 8, targeting Type 3 and Type 4 power supply devices (PSEs) and consumer devices (PDs) of 4-90 W and 3.84-71.3 W.
Thirdly, the IEEE 802.3bt standard provides many new features and functionalities, such as support for efficient 4-pair power transmission, as well as channel definitions for 2.5 Gbps, 5 Gbps, and 10 Gbps PoE operation.