An at-mention is the @name span you type in text. It refers to an Identity, and it has two directions: written plainly it addresses that identity, making it a Recipient, or when used as a signature it credits that identity instead, see Authorship.

@name syntax

Type @ followed by a name with no spaces, e.g. @ada. While typing, autocomplete offers every known identity.

A name may contain dots, so dotted usernames carry whole: @pete.smith and @ada.b.lovelace are each one mention.

Any name is a valid mention. Nothing needs declaring up front: mentioning @sales makes sales a known identity. See Identity for more info.