In all measured indicators the numerator is the debt service annual amount in current US dollars. Its exact "official" definition runs as follows:
Total debt service is the sum of principal repayments and interest actually paid in foreign currency, goods, or services on long-term debt, interest paid on short-term debt, and repayments (repurchases and charges) to the IMF. Data are in current U.S. dollars. Source : World Bank, Global Development Finance.
The debt service / tax revenues refers to the ratio between the debt service annual amount in current US dollars and the tax revenues during the same year. Tax revenues are defined as follows:
Tax revenue (% of GDP) comprises compulsory transfers to the central government for public purposes. Compulsory transfers such as fines, penalties, and most social security contributions are excluded. Refunds and corrections of erroneously collected tax revenue are treated as negative revenue. Data are shown for central government only. (Source: International Monetary Fund, Government Finance Statistics Yearbook and data files, and World Bank and OECD GDP estimates.)
DS/TR = Total debt service (current USD) / [(Tax revenue (% of GDP) / 100) • GDP (current USD)]
A ratio DS/TR = 0,457 means that the debt service represents 45,7% of the tax revenues. A ratio DS/TR = 3,487 means that the expenses due to debt service is 3,487 times more important than the tax revenues.
Mode of interpretation: DS/TR measures the burden of the debt service in relation to the country’s tax revenues. It highlughts the extent to which debt service hampers debtor countries in the use of their financial resources.