The advertiser identifier is a number, GUID or any other kind of ID determined by the supplier of the inventory management system and uniquely identifies advertisers. When sending objects to DV it is necessary to include the advertiser identifier (either in the data record or for instance in the file name of the batch) so DV knows under which account to advertise the object.
The advertiser does not need to be the actual owner of the object. An object can for instance be (temporarily) advertised by another branch of the same company. When using incremental updates and moving an object from one advertiser to another two mutations need to be sent, one delete mutation for the old advertiser and one add mutation for the new advertiser.
Each advertiser has a unique automatically generated Hexon identifier as well. These are not used in communicating with the supplier of the inventory management system.