By default, customers are assigned public pricing. In Customers > Cloud providers, the default selection is the master price book. The master price book lists each cloud vendor's current, publicly published pricing for the underlying SKUs (typically, thousands of SKUs) that are in the cloud providers' price books.
The SKUs in the master price book include all the products and services that the cloud provider bills for.
In the master price book, you can browse to the lowest-level metric (a SKU) that has a price associated with it for any usage.
When you create custom price books, you can think of the master as a sort of pivot point or template.