If an administrator does not enter the domain name when logging in to the device, the administrator is added to the global default administrative domain. If the system administrator has created a domain name for user authentication, for example, localuser, users must enter their user names with the domain name delimiter and domain name (@localuser). This reduces operation efficiency.

To facilitate operation, the system administrator can use the default-domain admin command to specify localuser as the name of the global default administrative domain. Then users do not need to enter @localuser when logging in.

<HUAWEI> system-view
[HUAWEI] aaa
[HUAWEI-aaa] domain localuser
[*HUAWEI-aaa-domain-localuser] quit
[*HUAWEI-aaa] default-domain admin localuser
[*HUAWEI-aaa] commit

You can also find the answer in Common AAA Operations