The order in which cells are numbered for UnStructured grids is as follow.
1.CellID 1 starts on the model boundary (polygon), in the upper-left corner. Cells are then numbered for all vertices on this model boundary polygon, going in a counter-clockwise direction
2.After this, the add-ins are numbered, starting with polylines, polygons. Vertices are the shapes are numbered based on their order, then points are included.
3.After this, the remaining cells are numbered, filling in the gaps, starting with layer 1, and working downwards.