CAP493, but I count find it in CAP413
3.4.2 When transmitting messages containing the following information, each digit shall be transmitted separately:
• Aircraft callsigns;
• Altimeter settings;
• Flight levels (with the exception of FL100, 200, 300 etc. which are expressed as 'Flight Level (number) HUN DRED');
• Headings; • Wind direction and speed; •
Pressure settings; • Frequencies; • Transponder codes; • Aircraft speeds.
