Additional income types are the payments you make on top of your employee's regular pay, so examples of these income types would be expenses, commission, bonuses, etc.
You can now add these income types directly from the payroll timesheet for both your hourly and salaried employees, at the time you are processing their payroll.
The drop down will include all the income types you have set up for your company, so if any income types are missing from the list, you can add those via the Company > income tab.
Any of the income types you add on the payroll timesheets directly will assume that it a one-time payment, payable only on this payroll.
To set up any recurring payments, you will still need to do that by setting it up in the "Income" tab for individual employees.
Any income types that have already been set up as recurring payments via the Employee > Income type tab will show up as a column in the payroll timesheets. You can edit those amounts directly from the timesheet now instead of going into individual employee accounts and making those updates.