This is very useful in the most of situations: if you need to add or modify one ore more fields in cards based on one template you just modify the template. At any time you can import any predefined template or create your own.
But in some specific situation you like to add or modify any count of fields in your card individually, not in the all cards based on the same template. To do this you should “unlink” the card from the template. Just start to edit your card and tap on the “unlink” icon:
After that you can edit fields structure individually in this card: