Refer to figure.
EASA learning objective 022.11.04.01.01 states: ‘Describe the purpose of a CDU/MCDU. ’
EASA learning objective 022.11.04.01.02 states: ‘Describe the typical layout of a CDU/MCDU and the general purpose of the following: screen; line select keys; menu select keys; alphanumerical keys.’
EASA learning objective 022.11.04.01.03 states: ‘Explain the function of the ‘scratchpad’ part of the screen.’

Pilots interact with the flight management computers FMC) of the aircraft flight management system (FMS) using a control and display unit (CDU) or, as Airbus calls it, a Multifunction Control Display Unit (MCDU). The CDU/MCDU is what's known as a Human-Machine -Interface. Usually two CDU/MCDU's are installed, one for each pilot which typically allow the following information to be inserted using its alpha numeric keyboard or displayed on its screen:

• Insertion of a flight plan using waypoints, company routes and ATC departure and arrival procedures
• Selection of pages for data insertion or display of information
• Data insertion (speeds, weight, cruise level etc)
• Selection of navigation functions ('Direct to' etc)
• Interface with datalink systems

The CDU/MCDU display screen displays pages of information which all have titles to explain their function. These pages will have selectable lines which provide the pilots with insertion and display options.

Along the left and right sides of the screen are Line Select Keys (LSK). The LSK's allow the pilots to select the option displayed on the screen alongside these keys for the purpose of data insertion or to select another page. Information inserted via the alphanumeric keyboard is displayed on one line at the bottom of the screen, this is called the scratchpad. The purpose of the scratchpad is to allow pilots to check data before it is inserted or for the FMC to display messages to the pilot. For example if entering the required cruising level the pilots would type ‘FL330’ which would appear in the scratchpad, having checked that it is correct the pilot can then insert it using the appropriate LSK alongside the blank field on the display screen.