PromoMats uses matching fields to match Claim records to documents. All standard and custom object reference and picklist fields (except Category) can function as matching fields if they exist on both the Claim object and the Document Type.
When trying to create a custom picklist or object field in the Claim object, an error occurs: Number of Picklist and Object fields  exceeds [max:5]. Why does this happen?
In Vault, the Maximum number of matching fields is 5. This means that the number of custom picklists and object fields to be created is set at 5. If an admin tries to create the sixth custom picklist or object field, the error occurs.
Contact Vault Support to increase the Maximum number of matching fields.
Vault Documentation: Vault Help