I think what they mean is no where in the regs is duty-time limit defined. The only thing that is mentioned is you must have _____hrs of rest within the 24 hrs preceding a duty period in which a flight is assigned, depending on which regs your company follows. Also to be noted, rest periods cannot be retroactively assigned, meaning the cannot be adjusted after the fact. You have to know when you are going on duty. By process of elimination if you are coming onto duty with 10 hrs rest you have a max availability of 14 hrs after your duty-on. This is because at 14:01 of duty you can no longer look back 24 hrs and see 10 hrs of rest. This example is of course for crews in an unscheduled operation.