What is the purpose of the value in the Merge Fields document field in Vault?
The purpose of this field is for diagnostic purposes upon initial document upload, as this field is required to be defined in the associated Document Type in order to be available on the affected document. If this field is not available for the affected Document Type or if this field is set to a No value upon document upload, then the merge fields on the document are not detected by Vault.
Document users do not need to see this field, as this field is automatically set by Vault when a document is uploaded. Admin users may, however, use this field to verify whether the Merge Fields setting is enabled for the affected document and that there are merge fields being detected on the document.
The proper configuration for this field should be either Hidden or Read Only. However, this field can be set as Editable for Admin users for diagnostic purposes.
Vault Rendition Documentation: Using Merge Fields for Microsoft Word & Excel