Applications register themselves with the LDS on the same host if they wish to be discovered. The registration ensures that the applications is visible for local discovery (see 4.3.4) and MulticastSubnet discovery if the LDS is a LDS-ME (see 4.3.5).
The OPC UA Standard (OPC 10000-4) defines a RegisterServer2 Service which provides additional registration information. All Applications and LocalDiscoveryServer the shall support the RegisterServer2 Service and, for backwards compatibility, the older RegisterServer Service. If an Application encounters an older LDS that returns a Bad_ServiceUnsupported error when calling RegisterServer2 Service it shall try again with RegisterServer Service.
The RegisterServer2 Service allows the Application to specify zero or more ServerCapability Identifiers. ServerCapabilityIdentifiers are short, string identifiers of well-known OPC UA features. Applications can use these identifiers as a filter during discovery.
The set of known ServerCapabilityIdentifiers is specified in Annex D and is limited to features which are considered to be important enough to report before an application makes a connection. For example, support for the GDS information model or the Alarms information model are Server capabilities that have a ServerCapabilityIdentifier defined.
Registration with LDS or LDS-ME is illustrated in Figure 1.