This help doc will cover using add-on merchandise on an assigned seating event. With this setting, event-specific merchandise can be added automatically to a seat on a seat map. The user does not have the option to add the merchandise (it is included and pre-defined). 

A promoter might use this option if a seat came with a pre-defined merchandise item and they want the item to be listed separately in reporting vs simply including the item in the price of the ticket.