The aircraft type is decided by the Network Planning (or similarly named) team when they are setting the schedule. The type may be changed at any point while the flights are available for sale, but normally not without good reason. At this stage and up to 72 hours prior to departure, flights are said to be under "reservations control" and the primary means of managing them is the Computer Reservation System. At this stage the flight is sufficiently far away in time that any potential operational issues generally are unforeseeable and so no specific airframe/tail number will be allocated.

However Network Planning will have an idea about which sub-fleet of aircraft within a type are likely to rotate among a selection of routes and they will plan their rotations on this basis. So you often see the same tail number appearing on a certain route again and again, all through a season and sometimes over many years.

At 72 hours prior to departure the flights move to "airport control" using the Departure Control System, which pulls details of all the passengers from the Computer Reservation System and prepares to accept them for travel by issuing boarding cards, printing bag tags and, when passengers reach the gate, loading them on. When flights move to airport control the airline will have a much better idea about which air frame/tail reg will operate. It is still not perfect though and can be subject to last minute changes. You only really know which tail reg is allocated when you get onboard.

Hope that helps!