Explanation:
To migrate legacy install base into Salesforce CPQ along with the contract, the Revenue Cloud Consultant needs to populate the Assets and Subscriptions objects. These objects store the information about the products and services that the customer has purchased and are associated with the contract. The Assets object tracks the physical products, such as hardware and software licenses, that have been delivered or activated for the customer. The Subscriptions object tracks the recurring products, such as software subscriptions or maintenance plans, that have a defined billing frequency and term. By populating these objects, the Revenue Cloud Consultant can preserve the history of the customer’s purchases and enable the features of Salesforce CPQ, such as renewals, amendments, and co-termination.

References:

Migrate Legacy Data to Salesforce CPQ
[Assets]
[Subscriptions]