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STL is a stereolithography file type. It is widely used for 3d printing, prototyping, and manufacturing from 3d models. STL models can be imported into many 3d applications, but they are usually very high poly and require specific standards for the model that other formats do not require. You would not see a STL model, for example, being imported into a game engine. The model specifications are vastly different. If you wanted a 3d model for 3d printing, then STL is typically the file format most 3d printing software and hardware recognize and will accept.
I think he means how to open a .rar file which is a compressed file and can be uncompressed by free software like WinRAR or WinZip.
Oh you may be right. 7zip is another free alternative.
