First, each Mac OS X Server install, like each Mac OS X client install, includes a local NetInfo domain. This domain, which exists regardless of the server’s Open Directory role, maintains machine data for the server as well as local user accounts (including the originating administrator account and the root user). For servers that are not configured as part of an Open Directory infrastructure (that is, those that are standalone servers), the local NetInfo domain stores all user, group, and other account records (as it does on a workstation). When users attempt to connect to a standalone server, they must do so using an account stored in its local NetInfo domain.
In an Open Directory infrastructure, servers are almost always bound to an Open Directory domain, or are Open Directory master and replica servers hosting a shared domain. For Open Directory servers, you will probably rely on records stored in the shared domain for any access or configuration. The same is generally true for servers that are bound to that domain, regardless of the services being hosted by those servers.
However, local user accounts can be used to access and administer the server (but not the Open Directory domain). Such accounts, including the root user (if enabled), are stored in the local NetInfo domain. Note that Open Directory domain accounts can also be granted permission to administer servers. Also, as with Mac OS X workstations, many processes that run on a server rely on local accounts designed specifically to run those processes and they typically also exist in the NetInfo domain.
For standalone servers, however, the records in the NetInfo domain are relied on whenever a user accesses the server. This is true if the server is a file server, mail server, web server, or any other server. Administration of the server, as well as access to resources stored on the server, is authorized based on the records in the local NetInfo domain.