If you want to reference other documents, you should most likely use a reference property and the input field type document reference. Sophora DeskClient's user interface will show you the Sophora-ID of a referenced document, while its UUID is saved. Therefore, if you want to configure a specific document as a default for the document reference field of a document type, you have to enter its UUID in the Default value field in the property configuration.
The field validation expression in the property configuration shows you a picker containing all document types available in your system. If nothing is selected there, every document type will be accepted in the reference field. However, you may restrict the applicable document types by selecting only the document types which should be used there. A referenced document can be opened easily by clicking on the label of the input field.
The default value in the property configuration:
The default value in the document: