Seats work in one of two ways, depending on the route.
Routes with seat selection. At checkout, each passenger's Bus Seat row offers Choose Seat, for a small fee shown next to it, or Automatic seat (free). Choosing opens the bus plan - taken seats are greyed out, and the one you pick is held for you. With the automatic option a seat is assigned for free when the ticket is issued, and it is printed on your ticket.
Free-seating routes. The checkout says Seat reservation not available: there is no plan to pick from and you sit in any available seat when you board.
After payment a chosen or assigned seat cannot be changed online. If you need a different seat - to sit together, or for accessibility - contact us or the operator (their phone number is on your ticket) before departure, and they will help where the bus allows it.