Some fields have non-productive areas inside, like lagoons and trees. You need to create a inner border inside your field so Atfarm doesn't calculate the average amount of fertlizer for your field with this areas. Also, your application card will ignore this areas, making the spreading easier for you.
To do this, do the following:
1) click on inner border on the navigation menu.
2) click with your cursor onto one border of the area you want to exclude.
3) trace along the area you want to exclude.
4) double-click on the first point you added.
5) click save field.