The foaf:sha1 property relates a foaf:Document to the textual form of a SHA1 hash of (some representation of) its contents.

The design for this property is neither complete nor coherent. The foaf:Document class is currently used in a way that allows multiple instances at different URIs to have the 'same' contents (and hence hash). If foaf:sha1 is an owl:InverseFunctionalProperty, we could deduce that several such documents were the self-same thing. A more careful design is needed, which distinguishes documents in a broad sense from byte sequences.