Quality of edge glued panels
A/B - a standard high type of quality, commonly used in furniture and stairs. "a" means that one side of the wood panel is clean, without any defects. It is allowed only small live knots. "B" means that the second part of the panel allows different changes in wood. It could be sapwood and bigger knots than on A side.
А/С - a type of quality in which one face-side is clean and the other side is going to a back or bottom part of the object (bottom of the table or step for example). On C side it is allowed sapwood more than on B side, dark knots and the lack of color selection.
If natural defects of the wood do not annoy you, you can choose B/C type of quality or Rustic. Glued shield of these two types consists different defects (sapwood, knots, color changes). However, in case of correct processing, these defects could be a nice addition.