Q:

Which rider's time determines result in team time trial?

I have read in an article about this year's UCI World Championships:

There are six riders per team, meaning a smaller size when compared to, say, nine for last July’s team time trial in Nice for the Tour de France. Also the time taken is usually on the fifth rider but for the worlds, with the smaller teams, the time will be taken on the fourth rider.

Are there some official rules about which rider's time determines team's result in team time trial? Or is it decided by organizer of each individual race?

A:

Each race can set its own rules for the number of riders on a time-trial team and the rider that will be timed.
From the UCI rules (Wayback Machine), Part II: Road Races, Chapter 5 (Team Time Trials), 2.5.001:

The number of riders per team is determined in the programme -
  technical guide - and must be at least 2 and no more than 10.
There are six riders per team for the world championships.

2.5.014:

The specific regulations for the event shall specify on which rider of
  a team crossing the finishing line the classification of teams will be
  timed for the finish.
The time will be taken on the fourth rider at the world championships.

