I think what Greg B meant was S4S which is lumber lingo for “surfaced four sides”. Lumber can be bought rough from the saw, surfaced two sides (S2S) where the faces are planned and the edges are left rough from the saw or from the tree (called live edge), or skip planned (lightly planned on the faces to help in reading the grain). His reference to S4S was in relation to planning and closely dimensioning all of the sides and edges as part of the process of building projects.
Back in the “good old days” when furniture was made with just hand tools it was normal practice to not worry too much about the surfaces that didn’t show unless you needed to do that as part of the process for that particular part. It was common to just surface and square an edge and face, and to use those two reference faces for laying out all of the features such as overall dimensions, mortise and tenon dimensions and locations and from which to judge conditions such as squareness. There are a lot of situations where the final dimensions don’t have to be all that close and the surfaces don’t have to be finely finished and you still can have a good looking quality piece. That is one of things looked at when determining whether a piece is an original antique or a modern reproduction. Modern manufacturers use different techniques to make furniture and they normally surface all of the sides and edges to pretty much the same degree. There are advantages to making parts S4S, especially with machine work, but it requires more effort with hand work and has fewer benefits.