I can't seem to find documentation on why we need this flag? (It's part of Marketo's default campaign setup) What are scenarios when this comes into play?
This will basically only filter by anyone who filled out the form and was acquired by the Program (versus people who filled out the form, but may have been acquired by a different Program). You might be able to think of this as "new names from this Program".
In other words, the first time we saw this lead was because of this Program.
It is crucial for FT (First touch) attribution and net new name counting.
So is it the first time that the lead has interacted with your content, as they may have been in your database before, but had no interaction yet?
Best practice is for everyone in your database to have an acquisition program; if you migrated data when creating your Marketo instance, those would generally be grouped as "pre-marketo" or some such instance. Therefore, for a lead to have an acquisition program that would represent your net-new leads that the program generated. Hope that helps.
For what it's worth, we had to run through a bunch of smart campaign updates to get our data to a fresh starting point (even a few months after our instance was launched). The sooner you do it, the better!
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