L.2.13.8 External Link

The corresponding content from externalLink1.xml.rels is

<Relationships …>
  <Relationship Id="rId1" Type="http://…/externalLinkPath"
    Target="file:///C:\Source.xlsx" TargetMode="External"/>
</Relationships>

This relationship indicates that the supporting workbook that C2 references resides on the local drive, at c:\source.xlsx.