This box will automatically be checked if the current row is a setup price row. Setup price rows will be automatically inserted directly after a part's invoice row, provided that a setup price is linked to the part.
Setup price rows are handled differently when it comes to statistics. When the statistics are updated, the setup price is saved to the statistics for the above part. However, in the Invoicing Log you will see the setup price specified in a separate column.
You can link several setup price rows to one part, but then the box Setup price row must be checked on the setup price rows inserted after the part's invoice row. This is to make sure that the statistics for the setup price rows are saved for the part.