In the table with product/service lines, there's a green button at the top right '+ Add Line'. If you expand the button, three choices are offered. One of them is 'Add Section with Title'.
If you click it, a 'Section Title' field will open, in which you can add the intertitle. If you now add a new product line, it will automatically appear under this intertitle. You can change the order of lines and titles by using the up or down arrows at the left side of a product line.