advertisers and agencies should be created as accounts and contacts as contacts, and the party modelshould be adopted to define the relationship between them.

advertisers should be created as accounts, and agencies should be represented by a custom field added to the contact object. Contacts should then be assigned to the accounts

advertisers and agencies should be created as accounts, with agencies listed as child accounts to the client accounts, with contacts related to those.

advertisers and agencies should be created as accounts, and a custom lookup field should be created to represent the client/agency relationship Contacts should be created under the accounts.