This sign is an informative sign indicating that the road will expand with an additional lane on the right-hand side. According to the traffic rules, when there are two or more lanes in the same direction of travel, the right-hand lane shall be used when the traffic rules do not require or permit use of the left-hand lane(s).

This is therefore the lane you should normally use. There are of course situations when it is permitted to use the other lanes, e.g. to ease congestion.