This blueprint tries to collect all information about how to implement real world units (in, mm) in a standard compliant way.
Even if the user switch everywhere the unit to mm, inkscape convert it to px unit and write it in the .svg file resulting inaccuracies.
All the units in svg drawing needs to be in px unit. The grammar in the SVG specification does not allow other units then pixel: http://www.w3.org/TR/SVG/paths.html#PathDataBNF
More information about coordinate system: http://www.w3.org/TR/SVG/coords.html
Examples: This is invalid in SVG:
<path d="M 0,0 L 30mm,0 L 30mm,30mm L 45mm,30mm" id="path7887" />
The correct solution would be to specify it in pixel and after tranform it.
More resource on the web
Related bug reports
#168753: SVG defaults to pixels (90dpi) despite default units Duplicates: #168516: Inkscape doesn't save page size units in plain svg #170738: preserve units
Viewbox related bug reports: #171203: Use viewBox to explicitly define a coordinate system
Related discussions in the past
On the inkscape-dev mailing list:
Although attaching parasite information to an svg element is not necessary, but maybe useful if additional information must be keeped about conversion inaccuracy. Please see this thread for more information: Attaching parasite information to an object (or group)