You can move an operation so that no cost is assigned to it. This is helpful when you want to remove a line item from the estimated cost to manufacture the part.
For example, the Costing tool identifies pockets and holes individually to manufacture with the appropriate tools. However, if you have a special tool to manufacture a collection of pockets and holes with one operation, you can charge a cost for the collection of features. You can assign the cost to one of the features, then place the rest of the features in the No Cost Assigned 
folder.
Another example is that you may want to disregard a feature in the part. For instance, if you have a base part that contains a feature casted in it prior to manufacturing, you can move that feature to the No Cost Assigned folder so it is not included in the manufacturing costs.