Some would say sub-domains are the 'third level' domains in the sense that they are simply "sub folders" under the domain root directory, normally use to organize your website content in different languages or different categories.
Abanye bangathi izizinda ezincane ziyizizinda ezingezansi 'ezingeni lesithathu' ngomqondo wokuthi zingamane "amafolda angaphansi" ngaphansi kwesiqondisi sezinsizinda zesizinda, esivame ukusetshenziswa ukuhlela okuqukethwe kwakho kuwebhusayithi ngezilimi ezahlukene noma ngezigaba ezahlukene.