Both ticket types are listed as secondary ticketing options, and the difference all comes down to the appearance. It's up to you on how you'd like to set it up!
Nested tickets are a subdivision of a "parent" ticket, assigned when setting the ticket up. When a guest chooses a quantity of the parent ticket, the nested tickets will appear beneath it.
Add-On tickets, meanwhile, can be applied to more than one ticket type. Once a valid parent ticket is selected, the available add-ons will be displayed.
Think of someone buying tickets for a fundraiser dinner. You might offer choices of meat and vegetarian options. Nested under the meat options would be fish, chicken, and beef. You then might want to have a dairy-free add-on which is a more generalized variant that could be applied to any of these other options to account for lactose-intolerant diners.
There are tons of different ways to customize your tickets to fit your event!