Google Tag Manager supports loading multiple containers on the same page. It's useful if you have multiple companies or organizations working on the same site, but for one reason or another (e.g. governance) you want to restrict access to your main container. In these situations, having the other party create their own container and adding it to the site is the best of bad options. Well, in Google Tag Manager 360, we now have Zones that make managing multiple partners’ containers quite a bit easier.