Refer to figure.
EASA learning objective 022.04.03.01.02 states: 'Identify the two types of attitude indicators: attitude indicator; attitude and director indicator (ADI).'

The Attitude and Director Indicator (ADI) is a development of the electro-mechanical gyroscopic attitude indicator. It incorporates flight director command bars and may also display localiser and glideslope deviation bars. To acheive this the instrument and hence the gyroscope must be electrically powered. As the attitude indication relies on the gyroscope being powered there must therefore be some indication to the pilot that the power has failed, this will take the form of a red flag.

Note: An ADI will usually have more than one red flag, loss of power to the gyro will be indicated by an 'OFF' , 'ATT' or 'Gyro' flag. If the the flight director is off there will be a 'FD' flag and loss of localiser of glideslope will produce 'LOC' or 'G/S' flags. Clearly if there's no power to the instrument all flags will be visible.