This option allows you to record a partial payment that happens at the same time as the sale. There are a variety of reasons why a partial payment might be applied, depending upon your business, but this allows you to specify the amount of the payment as well as to what account it should be applied. It is subtracted from the subtotal or total, depending on where it is placed in the grid.
To add this as an item, you will need to add a new item within the Items app, and choose Sales Tax.
If you do not see the Items app, you will need to get access permission for this app from your administrator. The Items app is found in the Accounting Lists Pack.