Usage
The Usage object is the store of actual usage of products and services of your organization. Usage is associated with a Product (product_id) and a ProductContract (contract_id) that wraps billing information about an instance of that product. 
The above structure allows for customers to record usage for their clients against multi use-cases under the same contract.
Example
Company A sells an Employee Verification Product to both Youtube and Google Voice. However, they have a contract with their parent Google and invoice them once.
Parent Contract: Google 
    Sub Contract: YouTube
        Product Contract: 
            - contract_id: xxxxxx
            - product: Employee Verification (`product_id`: yyyyyy)
    Sub Contract: Google Voice
        Product Contract: 
            - contract_id: aaaaaa
            - product: Employee Verification (`product_id`: yyyyyy)
Their usage data would therefore look like:
| date | contract_id | product_id | quantity | 
|---|---|---|---|
| 2024-1-1 | xxxxxx | yyyyyy | 123 | 
| 2024-1-1 | aaaaaa | yyyyyy | 12 | 
| 2024-1-2 | xxxxxx | yyyyyy | 56 | 
| 2024-1-3 | xxxxxx | yyyyyy | 4 | 
| 2024-1-3 | aaaaaa | yyyyyy | 7 | 
| 2024-1-4 | xxxxxx | yyyyyy | 8 | 
| 2024-1-5 | aaaaaa | yyyyyy | 9 | 
The above structure would reflect billables toward YouTube and Google Voice separately but would also allow roll-ups to the parent Google.