What does aliases mean in ISPmanager when creating a mailbox?

Aliases are additional email addresses that you can use for this mailbox.

Mail sent to one of these addresses will be automatically redirected to the user's mailbox. Type here several addresses, separated by a space. The main mailing address (Name@Domain) is not required.

All aliases work only for the domain of the main mailbox, so the domain part of the mailbox is not necessary to specify.