If the current element is to be derived from a valuelist, the 'Source' column will contain the URI for that valuelist.
For example, the column labelled 'Mode of Issuance' contains this URI in the 'Source' column:
http://RDVocab.info/termList/ModeIssue
Also, if applicable, the 'List Schema' column will contain the url of the schema for the list to be used. This will be a machine-actionable .xml file.
If the current element is not to be derived from a valuelist (ie. free text), then both of these columns will be empty.