Win by away goals explained
Elfsborg won leg #1 2-1 against Odd in Europa League Round Three.
Today, leg #2 takes place. The winner is based on the aggregate score of the two legs.
Obviously, if the aggregate score is NOT equal, the side with more goals wins.
But what happens if the aggregate score is tied at the end of regulation play?
In this situation, one tie scenario is that Odd scores 1 goal while Elfsborg score none. Thus, the aggregate score is 2-2. The tie breaker is which team scored more goals away from home. Since Odd scored 1 goal against Elfsborg at Elfsborg in leg #1 they win on "away goals."
What if Odd is leading 2-1 at the end of regular time?
This would mean the aggregate score is 3-3 and both teams would have scored 1 away goal. Thus, the game goes to extra-time to determine the winner. If still tied, the contest goes to penalty kicks until someone ones.
What if Odd is leading 3-2 at the end of 90 minutes?
This would yield an aggregate score of 4-4. However, Elfsborg would have scored 2 away goals while Odd only 1 and thus Elfsborg would be declared the winner on "away goals."
UPDATE: According to this ranking web page (take it with grains of salt I suppose), Elfsborg is #204 in Europe, Odd is #262, Randers (Elfsborg defeated in round 2) is #373, Lahti (Elfsborg defeated in round 1) is #306.
If one uses the UEFA Club Coefficient:
Elfsborg = 140
Odd = 328
Randers = 254
Lahti = 404
Thus, it would appear that Elfsborg "should" have won as two ranking systems have them as the "better" team.