The Used Stock Report is a simple count of stock used in a task over a given period of time. This count happens in real time and is based on the products added to the service tasks of any Task, based on the Performed task status.
To access it, click Insights and Reports > Used Stock Report. You can then use the filters to specify a date range and see all the stock used within that period. At the top of the table, the headers can be dragged in to create groupings (i.e. drag in the Name header at the top, then the Technician header after it to see all used stock and which technicians added them). Headers can also be clicked to sort that data point in ascending or descending order.