S2S stands for surfaced two sides and means that the two wider faces of the lumber (front and back) are planed smooth. The edges will still be rough.
S3S stands for surfaced three sides, literally S2S and SLR1E
S4S stands for surfaces four sides and is the same as S2S plus the edges are straight and parallel.
SLR1E stands for straight-line ripped one edge, meaning that one side is straight the other will still be rough. S4S requires S2S and SLR1E.
Lumber being processed is tallied PRIOR to ripping in accordance with NHLA and Industry Best Practices. Waste of 10-20% should be expected for ripped lumber, higher for S4S lumber.