Why does `%i` mean the Wireguard interface?

[manywrt]

I am reading up on Wireguard, preparatory to installing it to my OpenWrt router, and keep running into these lines (they are supposed to give you access to the peer's LAN):

PostUp = iptables -A FORWARD -i %i -j ACCEPT; iptables -A FORWARD -o %i -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
PostDown = iptables -D FORWARD -i %i -j ACCEPT; iptables -D FORWARD -o %i -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE

From reading about iptables I know that the -i option specifies the interface you want.

From a random Web page, I got the idea that %i refers to the Wireguard interface.

The question is: Why does %i refer to the Wireguard interface? Is this part of Wireguard grammar, or that of iptables, or the shell itself?

I have no formal training in computers, but generally try to understand something of what I follow off Web pages. If I want to be the kind of person that knows why %i means what it means, where should I begin? Study what first? (I'm sure this is a ridiculous question. Apologies.)

[jow]

This is a kind of template, the %i is interpolated by the program processing it, “wg-quick” in this case. It will replace %i with the effective interface name, like “wg0”, before passing the commands onto the shell.