WTF are "user units" in Inkscape?

If you go to File > Document Properties, there's a "Scale" textbox that says "in per user unit". So WTF is a user unit?

• graphicdesign.stackexchange.com/questions/84163/svg-and-viewbox-values
• wiki.inkscape.org/wiki/Units_In_Inkscape
  • "'px' is defined in SVG to be the same as a user unit"
  • "If a 'unit identifier' is missing or 'px', the length is interpreted to be in 'user units'."
  • "Guides, grids, and other editing aides should be defined in terms of 'user units'."
• wiki.inkscape.org/wiki/Units_In_Inkscape:_Document_Unit_Change
  • "Changing the Document Unit changes the relationship between the width/height and viewBox such that the initial user unit (aka 'px') is equal to the Document Unit. "

what are "user units" in SVG?

• wiki.inkscape.org/wiki/index.php/Units_In_Inkscape#Generic_SVG
  • "User Unit — A term defined by the SVG specification: a unit length in the current user coordinate system."
• inkscape.gitlab.io/extensions/documentation/authors/units.html
  • "The authors of the SVG specification solved this problem by introducing an abstract “dimensionless” unit called user units. The SVG1.1 specification 1 is quite clear about their definition: 'One px unit is defined to be equal to one user unit.' ... So whenever you read 'user unit', think 'pixel'."
  • (TODO — read especially the part about the "user coordinate system", that seems especially important)
• stackoverflow.com/questions/7087586/svg-1-1-what-is-user-unit-and-how-to-convert-user-unit-into-absolute-uniteg
• w3.org/TR/SVG/coords.html, Ctrl-F for "user units"
• developer.mozilla.org/en-US/docs/Web/SVG/Tutorial/Positions
  • "Without further specification, one user unit equals one screen unit. To explicitly change this behavior, there are several possibilities in SVG. ... "
• the difference between SVG "viewport" and "viewBox"
  • "The viewBox can be thought of as much like the viewport but with two extra features: it can 'pan' and it can 'zoom'."