Available in DESelect Segment Enable, Plus, Advanced.
In the Target Definition step, you will be able to choose between three types of Data Actions: Overwrite, Append, or Update, depending on your use case.
By default, the Data Action is set to Overwrite.
You will be able, of course, to change the Data Action, by simply clicking on the blue link Change Data Action as shown in the image above, and by selecting the relevant Data Action from the pop-up as shown in the image below.
For a better understanding of how each Data Action behaves, please see the below elaboration on each Data Action:
Overwrite: removes the existing records and adds the new ones (it doesn't require Primary Keys, but if you have them, it will automatically remove the duplicates at random)
Append: adds the new records to the existing ones (it doesn't require Primary Key, if you have it, make sure you deduplicate first, or else you'll have "Primary Key Violation" errors)
Update: from the new records, it checks if they exist already or not: if they exist, they are updated; otherwise, they are added as new (it requires Primary Key since that's what it uses to check the existence of the record)