Even though the data page of type Savable, if we use activity as save type, we need to write our own custom logic to save the data. I think the Obj-save on the data page is not possible, pega treated as read only page only but if we are use the database save as save type, pega will take care of saving the data to database. Generally If we want to process the data before saving into database then you can choose “Activity” option. Please see the below image and it is a OOTB rule.
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