Is this % reference for images or text?
In general the % is being calculated by the parent container of said element.
So if i have a parent container that is 700px wide and the child container is 70% wide - The child container will calculate 70% of 700px.
You can see here:
https://www.w3schools.com/cssref/css_units.asp
% is defined as the percentage "Relative to the parent element"
Hope this helps Marius and best wishes for 2022! 🌻 Richard