Think of it from the device standpoint. The device has no user interface. It is simply a box with an Ethernet port on it. There is no access to the switch console. No network administrator. It is at an employee's desk. They have been told that the Ethernet drop at their desk has POE enabled. The administrator has enabled for than the default power for the port. They bring the device and plug it into the Ethernet drop. The device independently needs to find out what power is available to that drop over the Ethernet connection using a protocol layer over Ethernet that the Cisco switch supports. Currently to acquire power from a UPOE switch, we are using the LLDP protocol layer. From what we see, LLDP only allows you to request power not get the maximum amount available with one command. Using LLDP, if you request more power than available, LLDP will return that no power is set. We then go and try again until we focus in on the highest wattage that we were able to request. The device would like to know the maximum amount of power available from the beginning so the device can request that amount the first time.
Consider this device as a power converter. This device is taking POE power and converting it to a different voltage for another device to use. This device can take a wide variety of power ranges. Knowing the maximum amount available on the port will allow the other device to use the maximum amount of power if it chooses.
