Hi Planck,
CE series processing mode: By default, regardless of whether the user carries the username for authentication access, the device will add the domain name and then send it to the Radius/HWtacacs server.
S series processing mode: By default, the entered user name does not have a domain name, then the original user name is sent to the server for authentication.
The CE is the same as the S series switch when the user name is not included in the user name. The user does not carry the domain name regardless of whether the user carries the domain name.
I hope it helps!