While there are many approaches to defining what an identity is, as far as the Network Identity Manager (NetIDMgr) is considered, an identity is the unique user identifier that is accepted by a network service. Each credential that is managed by NetIDMgr is assumed to map to a single identity. The collection of credentials that map to a single identity is considered to belong to that identity.
The default identity is the identity that will be used by applications when a specific identity has not been requested. The Kerberos v5 plug-in will mark the credential cache that contains the default identity as the default credentials cache for the current logon session.