Welcome to the forums :)
ID3 tags are basically meta data - "an informal standard" which have grown since their introduction in 1997.
Do you mean "Produced notice (P)"?
I can't find any specific info about this tag, but I doubt it is something you need to register for... it simply is a date:
(YEAR) SPACE (TEXT)
You can input the year the song was produced (my guess)...
You don't need to fill in all the ID3 tags only the ones needed.
Here's some more info:
Hope this helps