These fields are to set which worksheets the customer will have access to to be able to fill out in their customer login area. Simply select the primary worksheet that will always go with this event type (if any), and if you have any additional worksheets that typically go with that specific event type, select them in the "Additional Worksheets" section.
Before you can select a worksheet you must create one. To create a new worksheet, simply click "setup", then click "Event Planning", then "Worksheets". In the top right of your screen, you'll see a button to create a new worksheet.
Note: To select multiple worksheets in the "Additional Worksheets" section, simply hold CTRL and click on each worksheet that you want to select. Holding CTRL will allow you to select more than one additional worksheet.