If you do not select a seat on a United Airlines flight, the airline will automatically assign one to you. For a standard "United Economy" ticket, you can usually pick a seat for free at the time of booking; if you skip this, a seat will be assigned at check-in or at the gate. However, if you have booked a "Basic Economy" fare, seat selection is not included for free. In this case, United will assign you a seat randomly just before boarding begins, and you have no control over where you sit. Families or groups on Basic Economy tickets are frequently separated unless they pay a "seat assignment fee" in advance. If the flight is very full and you haven't been assigned a seat by the time you reach the gate, you may have to wait for the gate agent to finalize the manifest, but you are still guaranteed a seat on the plane unless the flight is overbooked and you are specifically notified of a change in your status.