If operat is set to -sum, results for the nodal data_item are summed. If operat is set to -average, results for the nodal data_item are averaged. For example, you can take for data_item the -node_rhside data item. In this way you can sum the external nodal forces on a part of the domain.
If operat is set to -moment and data_item equals node_rhside, the moments Mx, My, Mz are determined. These are moment with respect to the three model axes, each one in the positive direction of its axis.
This operation is done for nodes which are placed on the geometrical entity geometry_entity geometry_entity_index. Instead of a geometrical entity you can also use -all to tell that all nodes should be used. The result is put into the post_node_result record (with the same index).