A user's login name is the identifier they use to sign in to the platform. It is usually their full email address, and can also be referred to as the username.
For manually created users, the login name is set during user creation. For users from a directory (like Entra ID or LDAP), this information is passed automatically from the identity provider.
You can also set an alternate login name (often a shorter version, like john.smith instead of john.smith@example.com) to allow for quicker sign-in.