The eccCollection field is part of the EncryptedFieldConfig structure, which in turn, appears in the schema field of the EncryptionInformation structure. Currently, QE writes require that the eccCollection (as well as escCollection and ecocCollection fields) be present in the command's schema. This assertion is done in EncryptionInformationHelpers::getAndValidateSchema().

In V2, since ECC is removed, the requirement that eccCollection be present must be removed.