A technique includes deploying a string that includes a seat assembly in a well; and running a shifting tool in a passageway of the string. The shifting tool shifts the seat assembly to cause the seat assembly to transition between a first state in which the seat assembly forms a seat that is adapted to allow an untethered object communicated in the passageway of the string to pass through the seat assembly to a second state in which the seat assembly is adapted to catch the object to form a fluid barrier in the string. The fluid barrier is used to divert fluid in the tubing string to perform, for example, a stimulation operation.