This diagram illustrates the hierarchy used in a CGNS dataset to define a single boundary condition.

At the top level is a box labeled "BC", representing the boundary condition. At the next level are:

Below each "BC data set" box are the following three boxes. Below the "Dirichlet BC data" box are two boxes labeled with the Greek letters alpha and beta, and below the "Neumann BC data" box are two boxes labeled with the Greek letters gamma and delta.