1. Adobe Experience League, Workfront Documentation. In the article "Create and manage billing rates," the introduction explicitly lists the objects for which billing rates can be created.
Reference: "You can create billing rates for the following items in Workfront: Companies, Job Roles, Users."
Source: Adobe Experience League, Create and manage billing rates, Section: "Create and manage billing rates".
2. Adobe Experience League, Workfront Documentation. The same document details how to create date-effective rates, which is a property of the billing rate record that is then associated with the supported objects.
Reference: The section "Create date-effective billing rates" explains the concept of having multiple rates for different time periods, a feature applicable to the billing rates associated with the objects listed above (Companies, Users, Job Roles).
Source: Adobe Experience League, Create and manage billing rates, Section: "Create date-effective billing rates".