A good example is Materials. Instead of listing every single material used throughout the project you just want to show one line item called materials.
The Re-Entry is added to a specific view, and if you want to invoice it you have to add it to Final/Invoice.
From the Eye menu select add Re-Entry, then select the view filter you want to add it, select the category you want to group together, give the Re-Entry a name, then click ok.
Lastly you have to exclude the Materials from the View filter to avoid them being invoiced twice (once through the actual material and once through the Re-Entry) Open Show View filter from the Eye Menu.
The window appears at the bottom, click on Final Invoice on the list of names and then in the source column untick Locked. Then untick Project Materials from the same column and from the category column untick Materials.
Once all this is done the project window will have created a line item with this name and every time a material is added in the Time Report or the Project it will be grouped together.
Click on the White Circle icon to see the details of the Re-Entry in the Materials view.