Claimants must also be actively looking for work and, in most cases, unemployment has to be involuntary, although this may be difficult to observe in practice and the definition varies across countries. Benefit durations are limited in most, but not all countries. Insurance is mandatory for most employees, but voluntary in some Nordic countries. Job searchers whose entitlement to unemployment insurance benefits has expired, or whose work record is insufficient to make them eligible in the first place, may be entitled to unemployment assistance. In some countries, unemployment assistance is the main unemployment benefit.