Once you have defined a draft you want to use the next step is to provide mapping between the merge fields from the draft and columns in the source sheet. Basically you need to tell the add-on how each merge field should be processed - as a text value, or as a link, image or QR code. And this is the place where you have to specify it. Each merge field must be mapped either to a column or as a dynamic value.
When mapping a merge field you can select either a source column or a dynamic value. The drop down list contains all column names defined in header and as the last option is dynamic value.