TBills / Cash
Tbills could equally be called “cash”. They consist of very short term treasury bills with a maturity of a few months up to one year, and because of the direct backing of the government they are considered the single safest way to hold cash. The models on the site use the annualized 3-month Tbill rate of the country in question.
Tbills are a little different than most bonds in that they don’t have a coupon payment. They simply sell at a discount to their ultimate redemption price.
While popular historically, in today’s very low interest rate environment a common substitute with slightly higher interest rates and only a little more risk is short term bonds.