IEEE 802.3af (PoE), also known as Type 1, was the original standard defining power delivery over Ethernet. It supports a maximum power output of 15.4 W at the Power Sourcing Equipment (PSE), which encompasses Class 3.

IEEE 802.3at (PoE+), or Type 2, increased the available power to 30 W at the PSE. This standard introduced Class 4 to handle the higher power requirements beyond the original 802.3af limits.

IEEE 802.3bt (4-Pair PoE) introduced Type 3 and Type 4, utilizing all four pairs of the Ethernet cable. It supports Class 6 (providing up to 60 W at the PSE under Type 3) and Class 8 (providing up to 90 W at the PSE under Type 4).

IEEE Computer Society. (2018). IEEE Std 802.3bt-2018: Amendment 2: Physical Layer and Management Parameters for DTE Power via MDI over 4-Pair. IEEE. Section 145. (Defining Type 3 and Type 4 power levels/classes).

Cisco Systems. (2022). Cisco Catalyst 9000 Series Switches - PoE and UPOE+ White Paper. "Table 1: PoE Standards." (Confirms 802.3af maps to 15.4W/Class 3, 802.3at to 30W/Class 4, and 802.3bt to 60W/90W).

Ethernet Alliance. (2019). Overview of 802.3bt - Power over Ethernet. Page 4, "Power Classes." (Details the specific wattage for Class 6 and Class 8 under the 802.3bt standard).