LDAP - Where do I need Objectclass Top

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LDAP

Hello everybody I have a big Question to you. I have been searching over hours and I can't find the answer to my Question. I got the folwoing LDIF script from my school and I can't understand why I don't need the Objectclass Top at the organisationunit litec as you can see here:

Containerobjekte:
dn: o=Litec
o: Litec
objectclass: organization
description: Linzer Technikum

dn: ou=IT, o=Litec
ou: IT
objectclass: top
objectclass: organizationUnit
description: Abteilung Informationstechnologie

dn: ou=ET, o=Litec
ou: ET
objectclass: top
objectclass: organizationUnit
description: Abteilung Elektrotechnik

Blattobjekte:
dn: cn=Admin, o=Litec
objectclass: top
objectclass: person
cn: Admin
description: Admin

dn: cn=Alfred Mair, ou=IT, o=Litec
objectclass: top
objectclass: person
cn: Alfred Mair
sn: Mair
description: Abteilungsvorstand

dn: cn=Herbert Jachs, out=IT, o=Litec
objectclass: top
objectclass: person
cn: Herbert Jachs
sn: Jachs
description: Lehrer

thank you for your answer
yours
alex