What is a .at used for?

This is the domain extension for Austria. Anyone can buy and use a .at domain.

However, they are usually purchased by Austrian companies and individuals, or by businesses which trade in Austria or are considering it.

You might also want to look at a .at domain if your name, company name or the term you want to register ends in 'at'. For instance, if www.bureaucrat.co.uk was taken, you could try to register www.bureaucr.at instead.