The Products and/ or Services list are what you offer to your Clients / Customers. This is what you sell, and what will generate the income for your Business. The items that make up this list are the items that will shows up as the Revenue or Income on your Profit & Loss Report or Income Statement. The Revenue or Income categories were set up previously in the Chart of Accounts.
Items or Services in this list will show up on the Client/Customer invoices. These items, however do not all need to show up on your Profit & Loss statement as they might all fall into one Revenue line.