For more detailed information for UK nationals in Austria, see Living in Austria
Registration certificate for EU/EEA/Swiss nationals required within four months of arriving in Austria. Available from the local Bezirkshauptmannschaft or Magistrat (the MA35 in Vienna). Can be replaced upon application after 5 years of continuous legal residence with a Bescheinigung des Daueraufenthalts, although this is not compulsory.
Will cease to be valid if the UK leaves the EU without a deal but will be important evidence when applying for a new residence permit.