In iOS9 the context.evaluatedPolicyDomainState functionality has been introduced. This gives a 'value' for the registered fingers. In iOS8 this functionality was not available.
What happens for users who have registered their fingers in iOS8, and migrate to iOS9 (or even iOS10)? I assumet that the evaluatedPolicyDomainState function will return a 'value' for these users, and more important will that 'value' change when the user updates their finger (e.g. add a finger).
The evaluatedPolicyDomainState is an opaque structure that represents a the current "state" of the biometric database. If a device is upgraded from ios8 to ios9 then this state data will become available to your app the first time TouchID is used, but your app will have no prior value to compare against.
If the biometric database changes in any way (figures enrolled or removed) then a new "state" will be returned, but the state doesn't indicate the nature of the change and the state doesn't vary based on which finger was presented.
An app may choose to invalidate any cached credentials and require the user re-authenticate when the domain state changes as it potentially indicates that an additional fingerprint has been added to the database
