The different values of the international poverty lines represent different standards of what poverty means. The $1.90 a day (in 2011 PPP $) is the extreme poverty line and represents the poverty line typical of the world’s poorest countries. Other poverty lines for developing countries usually represents a slightly higher standard of living. The $1.90 a day poverty line can generally be used to measure poverty globally and to compare poverty across countries in less developed regions.
In more developed regions, higher international poverty lines are more appropriate.
For a detailed discussion of historical international poverty lines, see “Dollar a Day Revisited”.