In solidworks Help ,I find this explanation.
Why do it become this shape?
what usage for this function?
I'm not sure you actually got it. Translate Surface means that the new face is produced by translating the surface in the direction and by the amount you specify, say 5 mm in Y. Offset Surface means that the new face is produced by offsetting from the surface by the amount you specify. Suppose your surface has an outward bulge with a 5 mm radius. You can translate the surface by any amount and not have any problems. You can offset it towards the outside by any amount without difficulty. But you can't offset it by 5 mm or more inwards without failing.
I got it . if transtlate surface selected,it means that a smooth surface produced with caculation. if translate surface cleared, it means equal distance offset produced and the surface isnot "curve smooth".
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