Refer to figures.
EASA learning objective 022.04.05.01.02 states: 'Using a block diagram, list and explain the function of the following components of a remote-reading compass system: flux detection unit; gyro unit; transducers, precession amplifiers, annunciator; display unit (compass card, synchronising and set-heading knob, DG/compass/slave/free switch).'

The remote reading compass (RRC) is a directional gyro which is slaved to Magnetic North using the input from a flux detector which senses the Earths magnetic field. The annunciator unit indicates to the pilots that this synchronisation function is taking place and also provides a facility for manual alignment of the gyro. The display window shows a dot or cross to indicate which direction the gyro is being slaved, in normal flight this will slowly oscillate between a dot and a cross as the system corrects the gyro for drift. A spring loaded switch is provided for occasions where rapid manual synchronisation is required such as the initial alignment on the ground or after extreme manoeuvering. This is the normal mode of operation for the RRC and is called SLAVED mode.

There are occasions where magnetic heading are unrealiable, such as near the magnetic poles. The RRC has the facility to disconnect the slaving circuit to allow the RRC to operate as a simple directional gyro (DG). This is known as FREE mode and the annunciator unit will have a two position switch labelled FREE/SLAVED or DG/COMP to allow the pilots to select the appropriate mode.

It is therefore correct to say that the annunciator is used when synchronizing the magnetic and gyro compass elements