Bell nozzle

In terms of a momentum vector, there is an angle between the axis of the rocket engine and the gas flow.

The bell or contour shape is designed to impart a large angle expansion for the gases right after the throat.

A properly designed nozzle will have these two sets of shock waves coincide and cancel each other out.

In this way, the bell is a compromise between the two extremes of the conical nozzle since it minimizes weight while maximizing performance.

[1] The most important design issue is to contour the nozzle to avoid oblique shocks and maximize performance.

Test firing of Space Shuttle main engine (whose engine nozzle is very close in shape to optimal)