Please see this thread, just a few threads down on the listing:
As in Dorico 4, you can set the default gain level for outputs in the Mixer via the Play page of Preferences, but this slider does now in Dorico 5 show you the effective gain level.
The Mixer has been updated in Dorico 5 to allow you to add more send channels, rename outputs, etc.
All of this information is provided in the Dorico 5 Version History. I recommend you read that if you have questions about what features are included in the new version.