Every dataset used by SIS knows which coordinate reference system its items are defined in. If you overlay two datasets with different coordinate systems into a single SWD, then SIS will do "on-the-fly" conversions between them.
You can also change the viewing coordinate reference system with CRS [Home-Map].
The mathematics behind coordinate systems and coordinate reference systems can be quite complicated, involving spherical and elliptical trigonometry. This means that datasets will draw faster when their coordinate system matches the viewing projection. To speed up drawing, SIS will use a matrix approximation when the difference between a full conversion and a matrix conversion is less than a couple of pixels. (This approximation is only ever used for drawing, never for snapping or transforming items.)
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