What, then, is “Inkscape SVG” as opposed to “Plain SVG” when saving a document?
Inkscape SVG files use the Inkscape namespace to store some extra information used by the program. Other SVG programs will not understand these extensions, but this is OK because the extensions only affect how the document is edited, not how it looks. Extensions must not cause any rendering problems in SVG-compliant renderers. However, some non-compliant renderers may have trouble with the presence of the extensions, or you may want to save some space by dropping the Inkscape information (if you’re not planning to edit the file in Inkscape again). This is what the “Plain SVG” option is provided for.
