The @BLOCKROW function is used in conjunction with the branch-and-price (BNP) solver to assign rows to blocks.  The BNP solver requires that a model be partitioned into two or more blocks of mostly independent rows.  The BLOCKID argument is a non-negative integer, indicating the block index, while the ROWNAME argument is the name of a row, perhaps indexed, that appears in the model.  For more information on @BLOCKROW, please refer to the BNP solver documentation.