\(...\) is Vim's way to create a capturing group. It was listed under the atom entry in Vim's doc. There are two other forms of atom,
\z(...\). The document is not very clear on what exactly does they do. Could anyone help to explain it please?
/\z( seems to do the opposite, it exports a group found outside the context where it has been found. It seems to apply only to syntax highlighting. I've never used it.