"sap allowed" in walnut timber
It means sapwood (lighter colored) is permitted in the grading of the board,so it's not guaranteed to be a full-face of "black" walnut (which is the most common walnut to run into that terminology for.) If the's been steamed, the color may not be all that noticeable, since that migrates color into the sapwood from the heartwood - for air dried there is a striking difference (and I consider it more often of benefit than a defect.)