World Bank lending categories
As this page states:


  International Development Association (IDA) countries are those with low per capita incomes that lack the financial ability to borrow from the International Bank for Reconstruction and Development (IBRD). Blend countries are eligible for IDA loans but are also eligible for IBRD loans because they are financially creditworthy.


Therefore the classification of countries can be representd with the following Venn diagram:



The final category, IBT, compromises IDA + IDBR, equivalent to IDX + IDB + IBD.

Note: this is my interpretation, and not based on an official definition (which I couldn't find).