The parameters in the Stopping Criterion panel control the criterions which stop the piling process. The analysis of the results file (GDR file) shows any disparity between the achieved result values and the desired ones.

Besides the same options available for Create Grains, it is also possible to choose Fill to Rim.

Fill To Rim

Fill to Rim continues the piling process until the objects reach the top of the bounding box in the Z-direction.