krb5_principal_intro man page

krb5_principal_intro — The principal handing functions.  A Kerberos principal is a email address looking string that contains two parts separated by . The second part is the kerberos realm the principal belongs to and the first 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