This option will only work with sFiles Enterprise.  If this field is blank, the object will default to the org-wide Named Credential. This value is typically set by the setup wizard and should only be changed if you determine that a different named credential should be the default for internal users.

To change the value of this field, input the "Name" of the desired Named Credential.

NOTE: This field is case sensitive. Ensure the exact value of the Named Credential's Name matches the value in the Named Credential Name field.

If a value is not specified, sFiles will defer to the Named Credential Name specified in the SharePoint Settings.

To view or change the Named Credential Name, go to Salesforce Setup > Custom Code > Custom Metadata Types > click 'Manage Records' next to SharePoint Object Settings and select your object.