What does properties="svg" on an <item> element in the manifest mean?

[rtmyers]

Hello, does anyone happen to know what the properties="svg" attribute on an <item> element in the manifest of an OPF file actually does? Is is necessary in order for some readers to process the SVG? epubcheck complains if it's there and there's no SVG in the file, or if it's not there and there is, but in my case I am generating SVG at run-time, so I need SVG to be processed but there's none there in the file at the point in time when epubcheck is looking at it.