OPEN and SERVER are built-in logical servers that always exist; their membership is implicitly defined, and is never stored in the catalog.
OPEN Logical Server
A node that is not a part of any user-defined logical server is implicitly a member of the OPEN logical server. The OPEN logical server consists of all multiplex nodes that are not members of any user-defined logical servers.
SERVER Logical Server
On each multiplex server, the SERVER logical server has implicit membership to that server only, allowing certain privileged users to connect to any node of the multiplex and execute server-level administrative tasks without knowing which logical servers include that node.