When the mark required option is enabled you can either select one mark or multiple marks on the dashboard you want to add data. If you select a mark that does not match the worksheet configured on Write-Back you will be shown an error notifying you are choosing the wrong marks! If the mark required option is not enabled then there is no need to select a mark (see below "Add Data").
Be aware that while selecting multiple marks there is a difference between drag selection and click selection. This is specially relevant when fields have been added to the details shelf because while they are not needed on the viz they still need to be captured them on Write-Back. The behavior is as follows:
In short, we are just trying to raise awareness of the differences between both selections because they will produce different results when using Write-Back to insert data.
Click Add Data
This mode is typically used when submitting data for the whole screen, and not a particular mark. For instance when you need to comment on a monthly dashboard. To be possible to add data without selecting at least a mark, you need to have the option "Marks required" deactivated from the configuration. In this case, just press the Add Data button.