Responsive margins and padding (Ethan Marcott - Responsive Web Design)?

Question

Adaptive design

Good day to all!
I read in Ethan Marcott's book - Responsive Web Design, these lines:
"The contexts for flexible margins and flexible margins are slightly different.
1. When setting flexible margins for an element, take the width of the element's container as the context
. for the context of the width of the element itself. Think of the "box" model and these sentences make sense: we describe the box in relation to the width of
the
element
itself .
That is, neither margin nor padding fulfills these words: When setting a flexible margin for an element, take the width of the element itself as context.
The div element itself has a width of 100px, while the margin is only 50% and it has 609px, which clearly indicates that these values ​​​​are taken from the width of the container block (which in this case is the body), and not from the element itself.
You can call this question in another way: how to correctly calculate margins and indents in adaptive layout?