I'm not quite sure what you're getting at. There is no "FIELD value," but there is a field called VALUE.
Remember, Power Ports are specialist symbols. Their function is to define a global net with the name of the desired voltage rail.
Open one in the Symbol (Library Part) editor.
In the part properties, there is a checkbox: "Define as power symbol." This is important.
Power symbols have one pin, of type Power In, not Power Out. Why not Power Out? Because it's not a source of power. If you placed two +12V power ports on a schematic, and their pins were declared as Power Out, the ERC would rightly complain: two outputs connected together. Obviously that's not what you want. So the pins are declared as Power In.
The power symbol Part Value is what differentiates one power port voltage from another. The symbol's pin's Pin Name should be the same as the Part Value, as it determines the name of the net to which the symbol is attached. Power Ports are global nets.
Of course there is no footprint associated with a Power Symbol. The netlist export from EESchema is smart enough to know that no footprint is associated with Power Symbols (see the "Define as power symbol" above), so when the netlist is imported into pcbnew, the power symbol isn't even present as a part. Again, it only defines a global net.
One more thing. You draw your schematic, and you place a power connector on the board and then you put a +5V Power Symbol on one of the connector's pins, and you place a GND power symbol on the other pin, and you use +5V and GND wherever they are need. And you run the ERC and it complains: "no power source" (or something similar). And that's valid. Why? Because the connector pin probably isn't declared as Power Output type (it's probably Passive) because connectors can be used any which way.
The "fix" for this is the Power Flag symbol PWR_FLAG. It is a specialist symbol with its one pin declared as Power Output, and the ERC knows that it is the "source" of the supply on that net. It doesn't define the net label, it is just a stand in for a "real" power source.