in
"7.3.5.7 Message header
The current MQTT transport mapping does not support message headers. Any promoted field or additional fields defined on the WriterGroup or DataSetWriter shall be silently discarded. Implementations shall not set the MQTT RETAIN flag, except for meta data messages published to the MetaDataQueueName as described in 6.4.2.4.6."

Limits the ability for the OPC application to use the RETAIN to publish to its own "domain specific" or persistent meta data to topics.

In this scenario, you have shared "meta information" with a long life-cycle, such as production settings for labels, that an OPC micro-service is responsible for. You want to be able to persist this in the MQTT broker as RETAINED. It should be the responsibility of the service (as it would be in the MetaDataQueueName topics) to make sure that the latest version of the message is retained by the broker.

RETAIN is useful for having systems with overlapping responsibilities, and non real time requirements, synchronizing in an eventually consistent way. They do not have to spam the topic or build a request response dependency, they can use RETAINED messages. This is especially true across domain and system responsibility boundaries.