The SALES ITEM NAME field is the name of the item that will be used on the invoice for your item sales. This will only be used if you to turn off the BY CATEGORY option. If you keep the BY CATEGORY option turned on, then the invoice will use an item for each category (from your point of sale system) that has sales.
On the invoice, the amount for this item will be the combined amounts of all your point of sale items. A sample invoice is shown below with the default clover SALES ITEM NAME highlighted in red.
If you do not have an existing item in your accounting system that has this name, then a new one will be created. The default account name for this new item is "Sales of Product Income" and the sub type is "SalesOfProductIncome".