krb5_principal_intro - The principal handing functions. A Kerberos principal is a email address looking string that contains to parts separeted by a @. The later part is the kerbero realm the principal belongs to and the former is a list of 0 or more components. For example

lha@SU.SE
host/hummel.it.su.se@SU.SE
host/admin@H5L.ORG

See the library functions here: Heimdal Kerberos 5 principal functions