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To link the document to another object, the ContentDocument Id field must also be present so that the value can be chained to Content Document Link mapping. S4S Cloud automatically re-loads the Content Version object to get the generated value when creating a new record.
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External file sources
Large files and those shared with other systems are better handled as external documents.
Salesforce Files Connect documentation
S4S Cloud can then be used to create the reference to the external document:
Perform the file upload to the external service using your website back end or as desired.
Submit the required information with the S4S Cloud form. Use a mapping that creates the Content Version object as above, but specify “E” for external Content Location and set the appropriate fields to reference the file in the external service.