The minimum required credentials for a connection to access a customer warehouse database is SELECT access, but should be elevated if the connection is used to manipulate data or create new tables.
The default database is used when executing SQL that does not specifically qualify the database name. Such queries can be executed using a SQL task in the Workflow, for example. Multiple connections can be created for the same server if the customer wants to use a different combination of credentials, query banding, or database values.