Our system is based on a hierarchy Account » Advertisers » Media Plans » Campaign Items. Settings that you apply at higher levels are automatically applied at lower levels.
As an example, a custom margin applied to one Advertiser is effective for every existing and new Campaign Item which belongs to the Advertiser; you do have the ability override the default margin on a particular Campaign Item or Media Plan. Margins applied at the Media Plan level will be applied to all Campaign Items under the Media Plan.