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.