A crop is added by capturing a crop type (cocoa, orange, oil palm, potato, etc.), a maturity (mature, seedling, etc.), a unit (hectare, stem, etc.) and a quantity. Should you link a crop to an actual polygonal geometry, then the quantity and unit fields are automatically populated using the polygon’s size. For your information, when using rates for crops, they are based on the combination of type, maturity and unit, and multiplied by the quantity.