When you open the procedure, the Create tab first appears. This is where you decide how to classify and select your list. There are a number of alternatives you can use to classify and select. As a general rule, the list will display quicker when classifying by Customer code than when classifying by Customer category.
There are also a number of list types to choose from. Under the heading Show... you can choose to include budget values if such have been registered on the parts in question. You can also choose to display zero values, in other words the customers to whom nothing has been sold.
Read more about the different list types under Printout samples.