Rules

Links that file themselves — by their address.

Plays from YouTube (youtube-nocookie.com) once you press play — until then nothing is loaded from Google.

A rule files a link by its address, so you rarely sort by hand. Rules apply when you save, and when the panel looks for the group of the page you are on.

Facets: github.com/* marks everything from GitHub as a repository, in whatever group you save it.
Groups: *.acme.example/* sends everything from Acme’s servers to Acme.
You are here: open a saved page and its group lights up; a page that is not saved but fits a group by a rule or by links nearby says Fits this page.
Make a rule right from the panel, for the page you are on.
Test a URL in Settings › Rules to see which rules it would hit.