Roughly what happens is:
You have tags inside a scanclass at rate X.
For OPC-UA tags in that scanclass a subscription is made on the server at rate X.
The tags in that subscription request to be sampled at rate X.
At this point it depends what UA server, driver implementation, etc… you’re talking to. Our UA server/drivers will then poll the devices for tags at whatever rate the tags have been asked to sample at.