Type
Internal
The internal “px” (aka “SVG user unit or SVG pixel) is the same than the CSS Pixel.
User unit: The length corresponding to the value one after accounting for any transformations
External
Some Inkscape templates set one internal “px” to equal a different external value such as one millimeter or one inch)
Management
Scale
The scale property define the ratio between the internal and external pixel
Ie Scale define the number of CSS Pixel (user unit) by screen pixel ?