In most races, especially recreational races and long-distance events such as Vasaloppet and other Ski Classics competitions, it is up to the organizer to decide whether the use of Thermotip is allowed or not. Considering that Ski Classics promotes Thermotip and that athletes in several Ski Classics teams use it, one can assume that this is not an issue.
In certain competitions that are directly sanctioned by FIS and included in the international competition calendar (e.g. the World Cup, Scandinavian Cup, Swedish Championships, and the Swedish Cup), however, FIS regulations apply. These include a rule prohibiting equipment powered by external energy sources, which in theory could mean that Thermotip is not allowed.
For the vast majority of skiers this is not a problem, since most races in Sweden are not covered by these regulations. If you are still uncertain, we recommend asking the race organizers for clarification before your event.
If you would like to read more about the rules, you can find them here:FIS ICR