“Another exercise that might be worthwhile is to look at the use of the term “(digital) identity” with subjects other than people. If such subjects (a shipment, a car part, a bank transaction, a bank note, a company, …) had digital identities (and some of them do, I would think), can the same definition be used?”
That is really one of the key reasons the definition talks about digital subjects rather than simply “persons”.
To follow Johannes’ proposed route of inquiry, let me start with a familiar example – a web site representing a company. We then need to ask if it make sense to think of the digital identity of the web site being expressed as:
“… a set of claims made by one digital subject about… another digital subject.”
This is a case with which we have had a lot of experience: it describes SSL certificates, where web sites (public subjects with omnidirectional identifiers) have a set of claims made about them by a digital subject which is a company (a certificate authority such as Verisign – again a public subject with an omnidirectional identifier).
Really what happens here is that a certificate authority claims (shucks – asserts) that it does ongoing due diligence to establish that a web site with a given key is operated by a company (or other organization) with a given organization name, location, and other identifying information.
The set of claims represents a digital identity (good or bad), and the relation of this set of claims to reality is a potentially complex evaluation. As an observer I decide whether or not and to what extent I am willing to place confidence in the claims or use them in some way.
The digital identity of a shipment is not so very different. The shipment has an omnidirectional identifier, and it likely also has both a manifest and a destination. We could model this from an identity point of view in several ways. We could say the identifier, the manifest and the relationship to the destination digital identity represent the digital identity of the shipment. This is a set of claims. Again, the relationship to the actual contents of the shipment is “in doubt”. Typically, the shipment may be inspected as it crosses borders or when it arrives at its destination to determine whether the claims are in fact true. Thus the digital identity may or may not represent the actual real-world object. The extent to which it does depends on the competence and honesty of the subject creating the digital identity by making the claims, and the extent to which the shipment (and the claims themselves) have arrived at the inspection point intact.
In terms of modeling, the manifest could also be modeled as a separate entity, with a different digital identity, so the digital identity of the shipment would consist of only two claims – the digitial identity of the manifest and the digital identity of the destinee. The extent to which things are decomposed this way doesn't change anything for those who believe in the miracles of recursion.
We could go on like this, but I leave most of the exercise to the reader. I'll jump directly to example of the bank note.
I can imagine a set of claims corresponding to a bank note (denomination and serial number) and made by the mint or treasury. I suppose they could be used in tracing cash transactions. But what is interesting is that bank notes are just a symbol for wealth, the symbol being easily exchanged for other symbols (e.g. a check, a bank transfer, bills of a different denomination or a credit transaction, to name just a few). And thus there is no need for bank notes in the digital realm – new symbols and new ways of using symbols work just fine.
Here’s a story by John Fontana on efforts to extend identity to the world of things, in which Drummond Reed of XRI fame and Marty Schleiff, a long-time identity architect at Boeing, talk about their work on identifiers. Identifiers are the simplest claim, but I know from discussions that both would agree that the reasons identifiers are important is because they open the door to relationships between identities based on richer claim sets.