Zone seating is a recent introduction in the secondary ticket market that borrows entertainment trends from Europe. The seating itself involves venues being geographically divided into various chunks according to a visual model. The chunks concerned are often dubbed "Zones" and randomly drawn and marked with varying creativity. (Zones could be labelled as A, B, C or GOLD, SILVER, BRONZE or any such system of categorization.) Event-goers are then given options to purchase seats based solely on their Zone location. (Some sellers will choose to specify seating location in greater detail in their ticket notes.)
All seats are guaranteed to be together unless explicitly stated otherwise in the seller notes on the ticket listing. Seats that aren't together must be advertised as being "piggybacked" or "split" above a certain purchase quantity.
Ticket sellers don't advertise seat numbers publicly for various reasons. This may be to protect their privacy (if they are season ticket holders) and/or to prevent double-booking of tickets by multiple customers. Specified ticket listings (seat by seat) would result in numerous customers buying the same exact seats at the same time.