Originally Posted by ORDinary

CDO= continuous duty overnight. It is just like it sounds--you sign in in the late evening, fly one leg, sleep in a hotel for 3-5 hours, fly one leg back and are then finished with your all-night flight duty period. You get your legal rest during the day then and can fly that evening if assigned.

Split duty is basically the same, but when the total length of the duty period is longer than allowed, they can split it per part 117, provided you are given at least 3 hours in a hotel in the middle. It is essentially the same thing.

These trips go senior.