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SVG stands for scalable vector graphic it is used by printers and cutters (plotters) as the images are comprised of straight lines, regular raster (jpg, bmp, gif) images are comprised of tiny square dots and are not good for using on a cutting machine.
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Yes there is a daily download limit, registered users have a higher download limit, the average user should not have an issue with the limit. These limits are put in place to keep people from abusing the system.
You can use these files on any kind of software and machine that supports vector graphics. These include but are not limited to hobby cutters like Cricut and Cameo and big commercial machines like Graphtec, Roland, Jaguar, etc.