What is a Reporting Object?
Reporting objects define which objects in a Salesforce organization are available for reporting. Typically, when reporting on Accounting this is the Reporting Balances object.
If a reporting object record exists for a particular standard or custom object then it can be reported on. So, if you want to report on Accounts and Opportunities in Salesforce CRM for example, you would have to create a reporting object record for each one.
When setting up objects in a Master-Detail relationship, you only need to create a reporting object for the Detail (child) object. Once this is done you can query fields in the Master object without having to create another reporting object for it. The query will not work in the opposite direction (Master to Detail).
Reporting objects are used in reporting data ranges and reporting filters.