There is another concept, that of a node. A domain represents a number of nodes, and you can navigate your way to a domain by knowledge of the nodename rather than the domain name. Thus a node is a sort of alias for a domain.
Nodes are used in versions of WebSphere Application Server for distributed platforms, but not in WebSphere Application Server for z/OS and OS/390. They are not used by CICS. However, part of the structure for support of nodes is built when you set up a new LDAP server for use by CICS. Since WebSphere Application Server for z/OS and OS/390 does not use nodes, the nodename is an optional parameter to the DFHBuildSNS utility, which under CICS builds the system name space.