What is the legal obligation regarding carrying snow chains on heavy buses (>3.5 tonnes) in Austria during the winter period?

1. Snow chains are entirely voluntary if winter tyres are fitted to all axles
2. Snow chains must only be carried if traveling above 1,000 metres altitude
3. Appropriate snow chains for at least two drive wheels must be carried on board from 1 November to 15 April
4. Snow chains are only required during the months of January and February