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. 

NOTE: This process only attaches the merchandise item to the ticket for online sales. If you’re selling through the box office, you’ll need to manually add the merchandise item to the order.