While there are many approaches to defining what an identity is, as far as the NetIDMgr framework is considered, an identity is the unique security principal that is identified by a service. Each credential that is managed by NetIDMgr is assumed to map to a unique 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 picked up by other applications as being the default. For example, an application that uses Kerberos 5 can use the credentials found in the default Kerberos 5 credentials cache. Therefore, the Kerberos 5 plug-in considers the principal that corresponds to the default credentials cache as being the default identity.