The main benefit of an NIS+ domain hierarchy is that it allows the namespace to be divided into more easily managed components. Each component can have its own security, information management, and administration policies. It is advisable to have a hierarchy when the number of clients you have exceeds 500, if you want to set up different security policies for a set of users, or if you have geographically distributed sites.
Unless there is a need for a domain hierarchy, not having a hierarchy can simplify your transition to NIS+. When all users were in the same NIS domain, they were directly visible to each other without using fully qualified names. Creating an NIS+ hierarchy, however, puts users in separate domains, which means that the users in one domain are not directly visible to users in another domain, unless you use fully qualified names or paths.
For example, if there are two subdomains, sales.com. and factory.com., created out of the earlier .com. domain, then for user juan in the sales.com. domain to be able to send mail to user myoko in factory.com., he would have to specify her name as email@example.com. (or firstname.lastname@example.org) instead of just myoko, as was sufficient when they were in the same domain. Remote logins also require fully qualified names between domains.
You could use the table path to set up connections between tables in one domain and another domain, but to do so would negate the advantages of having a domain hierarchy. You would also be reducing the reliability of the NIS+ service because now clients would have to depend upon the availability of not only their own home domains, but also of other domains to which their tables are pathed. Using table paths may also slow request response time.
In Solaris 2.6 and earlier, the NIS+ servers for each subdomain are not part of the subdomain that they serve, with the exception of the root domain. The NIS+ servers are in the parent domain of the subdomain they serve. This relationship of server to subdomain creates problems for applications that expect the servers to be able to get their name-service data from the subdomain. For example, if a subdomain NIS+ server is also an NFS server, then the server does not get its netgroups information from the subdomain; instead, it retrieves the information from its domain, which is the domain above the subdomain; this can be confusing. Another example of when a hierarchy can cause problems is where the NIS+ server is also used by users to log in remotely and to execute certain commands that they cannot execute from their own workstations. If you have only a single root domain, you do not have these problems because NIS+ root servers live in the domain that they serve.
In Solaris 7, the NIS+ server for a domain can be in the same domain it serves. This allows a server to set its domain name to the same as that used by its clients without affecting the server's ability to securely communicate with the rest of the domain hierarchy.