In the Data Format tab of a CSV or XLSX feed, you can build a template for how the feed should be formatted. Please refer to the documentation of the feed consumer to get blueprints on how to build your feed template, in most cases an example feed provides enough information to build your feed in PIM.
Within a feed, you can edit the 'DATA FORMAT' section in the tab (1). Here, on the left hand side, you can set basic settings for the feed output (2), on the right you can build the template for your feed by adding columns (3). To add a new column, press the '+ Add new column' button at the bottom of the right hand section (4). When you have completed your changes, you can save them by pressing the 'Save' button in the bottom right corner (5).
After clicking ‘+ Add new column’ a window will appear where you can choose a value to add to your feed, you can either choose a product attribute from the dropdown (1), or start to manually write a value in case you want a default value for all your products in the feed (2).
When the value has been chosen, you can choose what name the column should have (1) and whether the feed should fill in values for all products or only the parent or the variating products (2).