OPC Unified Architecture is highly decentralized and is mostly concerned with the standardization of the independent interactions between UA Applications (i.e. between Clients and Servers and between Publishers and Subscribers). However, as the number of Applications in a given system grows, there are advantages to having some information centralized and interactions that are uniform among all Applications in a system. For example, if a system consists of one Server and one or more Clients, it is reasonable for the Server to be configured with the usernames and passwords of all users that can access the Server. If instead a system has hundreds of Servers, then it becomes unmanageable for each Server to independently store and maintain the usernames and passwords for all users of the system. For scenarios like this, the Unified Architecture includes certain centralized, global components to provide consistency and alleviate administration burden.

Ideally all Applications should work with all the defined global services when they are present in a system, but Applications that wish to utilize a particular global service need to be designed and built to do so. Keep in mind that the use of the global services in a system is always optional, so Applications should not be written to require their presence.