It explains it perfectly if you know what a round is. Please let me know if the Tournament Rules (
http://elementscommunity.org/forum/index.php/topic,6997.0.html) thread is ambiguous about this. A round is a set of matches. A match is a set of duels. All normal matches consist of duels to the best of 3. Thus, you need to have three decks ready against every opponent. None of these three decks may share an Element.
For instance: round 1. thraexis11 vs. Higurashi.
thraexis11 plays a time/light deck. Higurashi plays a fire/life deck. thraexis11 wins one duel.
Next duel, thraexis11 plays a fire/time deck and is disqualified. Higurashi gets one point.
The match results are now 1-1.
Assuming someone wins, thraexis11 or Higurashi may play their time/light and fire/life decks again in round 2, against the next opponent. The counter is reset every round, so to speak.
Marks do not matter, nor do ability costs. At all. Only what Element the cards you play belong to.