The Domain part isn't necessary if you have only one DSC line, or if you don't have any naming conflicts between different domains. AD FullName values are accepted but they don't work.
However, you do have a good point. Some companies assign cryptic Windows ID's to users (like u230194zz). Since you cannot enter a local email or AD FullName value in this field either, how is anyone in a company like that going to be able to address coworkers? Learn by heart about a hundred user IDs? Use Excel to keep this list readily available?