Refining matters most for pads and other small areas, where a coarse mesh renders battered walls badly and can produce erroneous results, especially in the 3D view.
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If refining would produce more than four million pixels it is skipped and a message suggests a coarser value.
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The polygon tool exposes this as its Refine control, with Off, 5 m, 2 m and 1 m settings.