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Parenthesis can also mean that the move may not be necessary. For example, for the F permutation:
(U') R' U R U' R2 y' R' U' R U y x R U R' U' R2
So for this case, depending on what angle you see the alg from, that U' might or might not be necessary. The way I hold this algorithm is with the block of 3 lined up on the left. So if I am speedcubing and I recognize the alg from this angle, I will first do U' to put the block of three in the front. However, if I recognize this case and the block of three is already in the front, it would be dumb for me to line it up on the left by doing U, and then doing U' again. I hope this makes sense...