Hello Azm
You’re on the right track with your understanding, but there are a few details that I would like to add. The Maximum Power per PoE port is indeed the maximum power that each switch port can supply to a device. If you measure the wattage at the port itself, you will be able to get up to this value. This is also the value that is subtracted from the total power budget of the switch itself, when this wattage is drawn from this port.
On the other hand, Maximum Power to PD refers to the maximum power that can be delivered to the Powered Device PD after considering power loss due to cable length and other factors. This assumes that cabling follows all the correct standards (i.e. 100 meter maximum cable runs, use of correct Cat5/Cat6 cabling, connectors, plugs, and terminations etc). The power loss over the cable is due to resistance, which leads to a decrease in power as the length of the cable increases. So, if you were to measure the wattage received by a PD, the maximum you would receive is that value.
So, using your PoE+ example, the switch port can supply up to 30W of power, but due to power loss, heat dissipation, and other factors, the PD will receive a maximum of 25.5W.
I hope this has been helpful!
Laz