Select & Better Lumber is *usually* virtually clear on one side. By definition it must be at least 83-1/3% clear of defects on its best face.Sadly, hardwood lumber can't be guaranteed to be absolutely free of defects 100% of the time.
Instead, hardwood lumber is sorted into various grades that guarantee a minimum percentage of clear, defect free wood. Grades are determined by the size of the board and the percentage of clear wood in the board. Size of board matters? Yes - think about it. A large clear board is more desirable than a small clear board.
These grades are not invented by Woodworkers Source, instead the rules and guidelines are set for the hardwood industry by the National Hardwood Lumber Association. The highest, best grade is called "FAS" and promises to be 83-1/3% clear on the poorer side (lumber has 2 sides, only one side determines the grade).
However, in practice FAS graded lumber is rarely available to buy from lumber producers. Instead, what a lot of wholesale lumber producers do is mix the top three grades (Select, F1F and FAS) and call them "Uppers" or "Select & Better" and sell it that way.
Select & Better lumber is the best grade that's available day in and day out. And most of the time it qualifies as "Better."