'Mark and Period' (M+P) is an old collector's reference that originally was only referring to Imperial porcelain that was identified with the correct mark of its real period. Thus the porcelain should not only be Imperial, of Imperial standard, by Imperial design, and ordered by the Emperor, but should also be correctly marked as such. So, however we have needed to soften up that label a bit, we should be aware of its origin.
To be noted, all Imperial porcelain is not marked. All marked pieces are not Imperial. All Imperial porcelain is not made and fired at the Imperial kiln.
All porcelain made at the Imperial kiln and fired there, is not Imperial. Kiln is here used as an approximation of the Chinese word yao meaning ware and Guanyao, meaning Imperial wares.
I could do this the whole day.