Invoice Worksheets are the output of the Infoplus 3PL Billing module.  That is to say, they are generated by the 3PL Billing module, by combining activity data with Customer Invoice Templates.  See the Overview of the 3PL Billing Setup App for an overview of how Invoice Worksheets can be generated automatically by Infoplus.

In addition to the Invoice Worksheets that Infoplus automatically generates using your Customer Invoice Templates, you can also manually create Invoice Worksheets, supplying all of your own values and line items.