Data connections let you connect the variables in your model to data sources, to pull in historical data.
When a variable is linked to data, it will take its value from the data if such a value exists, and will fall back to its own formula if a value doesn't exist. You can use this to perform forecasts based on historicals.
Here's how to connect an existing variable to a data item:
Check the categories page.
If your data item is broken down by a category you can transform the data item by aggregating the category (Causal uses the variable's category aggregation function), filtering category items, or grouping by another category.
Often your chart of accounts is more detailed than your forecast.