Recently, I saw a math equation on GameDev.SE with $\|$ in it, and I was confused because this was new to me. I googled it, but being a symbol, nothing specifically on it came up in my results.
The equation is supposed to find if a circle completely contains another circle; the equation looks like this: $$d<\|r-s\|$$ in which $d$ is the distance between the two circles, and $r$ and $s$ are the two circle's radii.
I'm using this in a Fortnite simulator as I am trying to find if a player is outside of the storm circle to test if they are in the actual storm and should take damage.
The author of that post is confused about the use of $|\cdot|$ and $\|\cdot\|$. They're using $|\cdot|$ for the norm of vectors and $\|\cdot\|$ for the absolute value of real numbers. The usual usage is the other way around.
You'll sometimes also find $|\cdot|$ used for the standard Euclidean norm on $\mathbb R^n$ (but rarely for more abstract norms), and since $\mathbb R=\mathbb R^1$ is a vector space and the absolute value $|\cdot|$ is its standard Euclidean norm $\|\cdot\|$, it's technically also correct to use $\|\cdot\|$ for the absolute value of real numbers.
However, doing both in one post (using $|\cdot|$ for higher-dimensional spaces and $\|\cdot\|$ for the one-dimensional space) is very unusual and unnecessarily confusing.