This term belongs to the normative terminology.
It classifies woods according to objective criteria such as their appearance, strength and durability. It determines for each category the nature and number of defects, singularities or anomalies admitted on the face and back layers.

Classification of the NF B 50-002 Wood norm:

Premium oak (SC):
No sapwood or knots, uniform color.
Premier oak (PR):
No sapwood on the face layer and knots in limited quantity.
Natural oak (NA):
Sapwood accepted without limitation.
Rustic oak (UK):
Sapwood excluded, knots accepted.
A, B, C chestnut:
Depending on the amount of knots, A is the best choice.