FastClose supports multiple ERP connections, letting you connect to more than one instance of your ERP system from within the same environment. The most common reason for this is having a live (production) connection alongside a test connection — so you can develop and validate reports against test data before running them against real figures.
The connections filter within a FastClose report is used to select the ERP instance to be reported against:

When you have just one connection configured, it isn't even necessary to specify a connection however when you have more than one connection configured (eg: prod and test) and the connections filter is left blank, FastClose will prompt users to select a connection every time a new template is opened. This is by design — FastClose needs to know which instance to pull data from — but it becomes an unnecessary interruption for users who only ever work with the live connection and don't even know a test connection exists.
The fix is straightforward: restrict who can see the test connection so that it is only visible to the users who need it.
After creating the connection in the FastClose Administration interface:

Set permissions so it is only visible to users who genuinely need it (typically developers or super users, not general report consumers). To do this:
Click "Repository Explorer" on the left hand side:

and then click on "Connections" in the middle:

Click on the "Permissions" button to select the users or groups of users that you wish to grant access to the Test connection, for example:

Once permissions are saved, make sure those users are briefed that they will need to explicitly select a connection in the connections filter before running a report, rather than leaving it blank.
A less common scenario is where your organisation runs separate ERP instances for different geographical entities (for example, a UK instance and a US instance). In this case, adding the numbers together is often exactly what you want, to produce consolidated group reporting.
This consolidation behaviour is enabled by the MDB element of the licence key. When MDB is active and the connections filter is left blank, FastClose will automatically pull data from all configured connections and merge the results — giving you a consolidated view without any extra steps. If a user explicitly selects a specific subset of connections, FastClose will merge only those, which is also valid where a user wants to report on some but not all entities.
However, MDB does introduce a double countin risk in the prod/test scenario: if a test connection is visible alongside live connections, a blank filter will cause test data to be silently merged into your figures. A warning triangle is displayed fin the connections filter to flag this, but the best protection remains restricting test connection visibility via permissions, as described above.