The source (e.g. document, act, legislation, text, picture, painting, recording, obelisk) in which some entity (e.g. Person or Location) can be observed. If the source that you are describing is not digitised or is not available as URI, you can define your own resource of class `roar:Document` and use for instance the properties defined by `schema:CreativeWork` to describe it. The `roar:onScan` property can then be used on an Observation to specify the location of the Entity in the source.