Which object controls the setting to make the Reference Document Number field required for entry?

The setting that makes the Reference Document Number field required for entry is controlled by the document type. Document types in SAP are crucial in determining the character of a document and include various definitions and settings that dictate how the system treats those documents.

Document types help in categorizing accounting documents and define important parameters such as number ranges and field statuses. Specifically, they control which fields are mandatory, optional, or suppressed during the posting of an accounting document. By configuring the document type, an organization can ensure that specific information, such as the Reference Document Number, must be entered, thereby enforcing consistency and compliance in data entry.

In contrast, elements like document reference keys or posting keys relate to the nature of the transactions and the financial accounts affected but do not dictate field requirements directly. Document field status is relevant, but it primarily depends on the configuration associated with the document types themselves. Therefore, the correct answer hinges on the role of document types in the overall configuration of document settings in SAP FI.