I'm still trying to wrap my head around populations in contact builder and how they replace Root relationships. Can anyone explain what are populations exactly used for and what is the impact in using them in Journey Builder? My understanding with root relationships is that you'd use them when your contacts weren't in All Subscribers. Seems like that's not a restriction anymore? Any help will be appreciated!
My understanding is the following: The former root extension was a single master data table for all contacts. Email address, name, or any other fields that make up the basic contact information. The new populations model now allows to tell the system that the basic information might be different for certain types of contacts and that this information might be held in different fields.
The easiest way to think about it are consumers versus business contacts. Quite often those data types come from different source systems. In one the email address field is called "Email", in the other it's called "Office email". With the new population model you can now tell the marketing cloud to look for the correct field depending on the type of contact you want to address without mingling both contact types into one root extension and all the data processing magic required for that.
At least that's what I think it will be useful for. ;)