The Optional Items Report gives an overall summary each optional item and the total quantity of each of these items purchased.  In terms of practical application, this is the kind of report that you would use to get a quick high level view of how much  / stock / quantities of optional items sold, remaining, unpaid and so one.  So a handy one to know about if your event has multiple optional items and variants. 

The basic fields for this report look like this:

To learn how to customise a report, click here: