"Default Tables" are the simplest types of Catalogs that can be added to the MinusX context. They automatically load the entire table list from the selected database (and are split by the schemas).
The agent can only see the tables (and fields) selected in this list + any tables mentioned in the SQL query in the SQL Editor. You can select/deselect tables like so:
The Default Tables are initialized to "Smart Defaults" to avoid stuffing the context with unnecessary and unused tables.
Smart Defaults
They comprise of tables present in SQL queries you may have written in cards or dashboards historically. We notice that this is mostly constant for a user, and is a great place to start. If you're ever unsure what tables to include, "Smart Defaults" is a good start.
Technical Details
Default Tables, under the hood, are just a special case of Advanced Catalogs. Each table is considered an entity, and each field is considered to be a dimension. There are no special metrics, or additional instructions included.