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 per inhabitant refers to the ratio between the debt service annual amount in current US dollars and the total population of a country. The total population is defined as follows:
Total population is based on the de facto definition of population, which counts all residents regardless of legal status or citizenship—except for refugees not permanently settled in the country of asylum, who are generally considered part of the population of their country of origin. (Source: World Bank staff estimates from various sources including the United Nations Statistics Division’s Population and Vital Statistics Report, country statistical offices, and Demographic and Health Surveys from national sources and Macro International).
Mode of interpretation: the Debt Service per inhabitant measures the amount paid each year to service the debt for each inhabitant; it is thus a good indicator of its "social burden".