Domain User is also a Plesk server client. The only difference is that the Domain User is limited to a single domain and is not capable of managing matters that influence system's functioning (i.e.: limits and quotas for the domain). The Domain User is however able to manage mail accounts at the owned domain, create Certificate Signing Requests (CSR) or generate self-signed certificates. Other than that the Domain User is granted all the nice things that make life easier, such as interface, and some other things that the Client is allowed. Accessing the PSA through your web browser, you can:
PSA warns you of any consequences before allowing you to execute a major change.