There are two aspects, or levels, to identifying information in OPC UA PubSub.

The actual information being exchanged relies on OPC UA PubSub physical identifiers. Example of these are publisher Ids and dataset writer Ids. The physical identifiers are those that are actually used to establish connections with the message-oriented middleware, and are those that appear in the network messages transmitted by publishers and received by subscribers. Some physical identifiers are always needed at certain point - without them, OPC UA PubSub cannot work.

OPC UA PubSub also has a configuration model (which can reside in OPC UA Server or in a file), and the configuration model provides logical identifiers (mainly, names) for PubSub objects. The logical identifiers (OPC UA PubSub logical identifiers) are easier to work with for humans, and the configuration model has a hierarchical structure which makes it better to comprehend. The use of logical identifiers also allows the higher levels to accommodate to changes, because they provide a mapping mechanism and "shield" the consumers from the physical details. For example, a dataset writer Id (which is an integer) may change due to configuration changes, but as long as the consumer refers to the dataset writer using its logical name (which does not change), the communication can still be established without a problem. The use of this extra logical level is, however, entirely optional, and depends on the consuming application.

QuickOPC provides way to work with either identifiers, and combines them together into OPC UA PubSub Descriptors. The transformation from logical to physical identifiers is done "behind the scenes" using a process known as OPC UA PubSub logical resolution.