Text in force at CS27-10: from CS27-0, effective 14 Nov 2003.
If the unusable fuel supply for any tank exceeds 3.8 litres (0.8 Imperial gallon/1 US gallon), or 5% of the tank capacity, whichever is greater, a red arc must be marked on its indicator extending from the calibrated zero reading to the lowest reading obtainable in level flight.
ED Decision 2003/015/RM · Source