The Band By option is similar to Group By: it is a second level of grouping in charts which applies colors to the points.
The Group By option means that one scatter/line point has the same value for the selected group-by field (e.g., if the field is Customer, then one point represents one customer).
If you group by Customer, and additionally band by Product, then every point is one combination of a Customer and a Product, so you get multiple points per customer, and multiple points per product. What is specific to Band By is that it drives the color of the point (color banding).
