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In QGIS 3.10 (Latest LTR) there are several unit types, see image below

example

What is the core difference between those units, in sense of their visualization behaviour on the Map?

What I noticed by so far, that some features do not always work with Millimeters, e.g. "Geometry Generator" and therefore you somehow need to switch from one unit to a different one.

On the other hand, you may work with a map, for example in a Print composer, and furthermore, such unit change can cause a huge impact on your visualization outputs.

Hense, I am looking for some kind of transforming strategy/approach that can lead me to the same visualization outcome but with the application of various Units.

Perhaps someone can refer me to any kind of explicit source with explanations.

References:

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    Okay, what is the core question here? If there is a relation/ratio between all these units? Or how their "visualisation behaviour" differs?
    – Erik
    Mar 5, 2019 at 10:08
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    I think the question of relation/ratio is rather self-evident, since except for the map units all these units are standardised. So, what do you mean by "visualisation behaviour"?
    – Erik
    Mar 5, 2019 at 10:14
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    generally speaking, there are two categories of units that you shouldn't confuse: let's call them visualization and geometry units. when you work on the geometry level (e.g. the Geometry generator), units refer to the underlying coordinate reference frame (e.g. units of CRS). when you are styling (visualizing) your layers, units refer to screen/canvas dimensions (Millimeter, Inches, Points); within that set of units, however, you can choose to have sizes in reference to the geometry level (Map units, Meters at scale) that are then translated to screen dims
    – geozelot
    Mar 5, 2019 at 11:02

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As was mentioned by @geozelot in comments

generally speaking, there are two categories of units that you shouldn't confuse: let's call them visualization and geometry units. when you work on the geometry level (e.g. the "Geometry generator"), units refer to the underlying coordinate reference frame (e.g. units of CRS). when you are styling (visualizing) your layers, units refer to the screen/canvas dimensions (Millimeter, Inches, Points); within that set of units, however, you can choose to have sizes in reference to the geometry level (Map units, Meters at scale) that are then translated to screen dims

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