Some options, e.g. control_mesh_macro, may generate tyings. Then the records node_tyings, node_tyings_factors and node_tyings_unknowns will be filled.
The unknowns unknown_0 unknown_1 ... of node index will be tied to the unknowns of the nodes node_0 node_1 .... The factor for node_0 is factor_0, the factor for node_1 is factor_1, etc.