as explained by Leo the PID means Product ID.
>> if PID is unspecified then what is meaning of that
if PID is missing either the module or component is not recognized by the system or the part is so dumb that it misses intelligence=cpu and a EEPROM, for example a FAN tray may be not able to show the part number.
another example could be a third party SFP optics in some cases.
Hope to help