External Systems define a relation to a source system. External Systems usually have stored the original document which will be one way synced to Livingdocs via Import.
An example:
externalSystems: [
  {
    handle: 'legacy-cms',
    label: 'Legacy CMS'
    url: {
      origin: 'https://legacy-cms.example.com',
      pathPattern: '/{{id}}/{{metadata.cmsId}}'
    }
  }
]
An External System defines an URL with origin and a pathPattern. The pathPattern can have placeholders which will be replaced with real values during runtime to build the final URL to the source system.
{{id}}-> replaced by LivingdocsdocumentId{{metadata.cmsId}}-> replaced by a metadata field with the handlecmsId