You can add logic to given fields by clicking on the pencil next to the field, you can then click "add condition" to add logic as shown below:
Clicking on add condition expands that section. You can now specify when the field is shown using one of three expressions.
All logic is controlled by values in a source field. A source field is a field that controls the behavior of the field you are editing.
There are three options to control when the field is shown.
You can specify complex logic by adding more than one condition to the field, controlled by multiple source fields.