Each domain registered must include contact details. These contact details are entered into a WHOIS database so that the domain owner can be contacted if necessary for various reasons. For domains registered with 1&1, your 1&1 account details are used for each domain by default. It is possible to change this so that different details are used for each type of contact.
The Registrant (Reg-c) and Administrative (Admin-c) contacts are the most important. The Registrant contact will be used primarily to determine who currently has rights over the domain for matters such as domain transfers. If the Registrant cannot be contacted, the Administrative contact will then be contacted.
Domain contact details can only be updated at the domain's Registrar. If you domain has not been registered with 1&1 (such as an external domain), you will need to update your domain's contact details with the domain's current Registrar. No contact details are used for subdomains.
Subdomains will use the "parent" domain's contact information. It is sometimes possible to opt-out of the WHOIS database so that the contact details are not publicly available.