Difference betwen Form Fillout and Flow change data value

Hi, Emmanuelle: Blocking updates to a field allows you to write to the field once and then retain the original value for the lifetime of the field. So in your specific instance: You have blocked updates to field Original Source in Admin. Then in a smart campaign, you want to change the data value of Original Source in your smart campaign's flow to "ABC." If a lead goes through that smart campaign and already has a value (any value) in the field Original Source, Marketo will not update the field for that lead. But for any lead who goes through your smart campaign, if they don't already have a value in Original Source, Marketo will update the Original Source field for that lead to "ABC." Thus preserving the value when it exists and capturing it for the first (and only) time when it doesn't already exist. More documentation on blocking field updates here: Block Updates to a Field - Marketo Docs - Product Docs

Hope this helps!