Database Connection Default and Override screens seem identical, but have different purposes.
This doc will explain the difference.
Something to keep in mind when using CleverPath Eureka Reporter is that the items on the Tools menu are generally global machine items.
Items on the Query menu are set for the particular iqr you have opened.
So the Query/Database Connection Override screen will allow you to save the data source, login and metadata information with the particular report or iqr that is currently open.
The Tools/Database Connection Default screen will set the default data source, login and metadata for all iqrs opened on this machine that do not already have a Query/Database Connection Override set.
So you can set the Tools/Database Connection Default screen once and have all of the iqrs that you create on this machine use this default information.
Or if you have an iqr that you want to use something different from other iqrs, you can set the Query/Database Connection Override screen and save this information with that iqr without affecting the rest of your iqrs opened on this machine.