Vernier thruster
The Vernier Thruster is a special case of the general type of spacecraft attitude control thruster, which is a smaller thrust motor than main attitude control motors and is used for fine adjustments to the attitude or velocity of a spacecraft.
Vernier thrusters are used when spacecraft control requirements, such as maneuvering during docking with other spacecraft, dictate a wide range of different thrust levels for attitude or velocity control. Main attitude control thrusters are used for larger movements, verniers for minor adjustments.
As an alternative, larger thrusters which can fire for very short durations or pulses may be fitted instead.