The $susy setting global-box-sizing doesnt change anything in Susy2 - susy

Question

This is something which I never understood in Susy 2.
Looking at this simple example shows:
http://codepen.io/itsthomas/pen/Btzxa
HTML:
<div id="wrapper">
<div id="content">
<div class="box1">Box1</div>
<div class="box1">Box1</div>
<div class="box1">Box1</div>
</div>
<aside>
<p>Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae.</p>
</aside>
</div>
SASS:
#import 'susy';
#include border-box-sizing;
$susy: (
container: 90%,
columns: 24,
gutters: 1/3,
/* global-box-sizing: border-box, */
debug: (
image: show,
output: overlay,
),
);
#wrapper {
#include container;
}
#content {
#include span(18 first);
background-color: red;
height: 100px;
padding: gutter();
}
aside {
#include span(6 last);
background-color: #F0A754;
padding: gutter();
}
.box1 {
#include span(6 of 18);
height: 40px;
background-color: #6D4214;
&:last-child {
/* margin-right: 0; */
#include last;
}
}
that adding the global-box-sizing: border-box, to the $susy map doesn't change anything at all, regardless if you use #include border-box-sizing; in your code or not.
The $susy setting global-box-sizing seems completely useless to me. Or am I overlooking anything?
Thanks

Answer 1

That's right. global-box-sizing is actually descriptive, not prescriptive. It tells Susy how you have set your global border-box. By default it is set to content-box (the browser default, and the border-box-sizing mixin will set it to border-box for you automatically. The only time you ever need to change the setting manually is if you set the global box sizing manually.
Susy needs to know what box-model you are using because it changes the math for certain grid types and functionality — anywhere padding and width might interact, like inside/inside-static gutters or bleed. You may not have any of these situations in your code, in which case that setting doesn't matter.