Intermediate bonds are a staple of most portfolio bond diversification. In addition, the treasury bonds used here are generally considered the safest form of bond because the government can always print more money there’s virtually no default risk. Even if you prefer a total bond market fund that includes corporate bonds, intermediate treasuries serve as a very close proxy in the calculators.
Intermediate term bond funds generally have an average maturity around 5-7 years. The returns on the site use a 3-10 year ladder that mirrors popular index fund methodology, and are based on the highest quality bonds issued by the treasury of the country in question.
Different countries have different names for treasury bonds. In the context of this site, treasuries, gilts, and bunds are all interchangeable.