3DS import options dialog box
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Merge Coplanar Faces
Planes can import into SketchUp with triangulated lines. Removing these lines manually can be tedious. The Merge coplanar faces option is used to have SketchUp automatically remove these lines.
The Units drop-down list is used to import 3DS geometry at the correct scale. A 3DS file has a field which indicates its units value. Sometimes this value is explicitly set in the 3DS file (such as millimeters) and sometimes it is missing and set to a generic value. The Model Units list item will import your 3DS file with the unit value it finds in your 3DS file. SketchUp will import your file using inches if a units value is not set in the 3DS file. SketchUp will import your 3DS file at the unit you choose if you pick anything other than Model Units.
For example, a face that is 1cm x 1cm will be imported into SketchUp as 1cm x 1cm if your 3DS file has its units value set to centimeters and you select Model Units in this drop-down list. A 1cm x 1cm face will import into SketchUp with dimensions of 1in x 1in if you choose inches in the drop-down list.