Distributed volume source terms (like force, heat generation, etc.) in differential equations. Here force_0 is the source term for the first primary unknown, etc. Use the control_print data item to print dof_label to get the sequential order of the primary unknowns. Specify source terms for each primary unknown. Consider the example with distributed volume force in x-direction for a 2D material:
force_element_volume 0 1. 0.
See also force_element_volume_factor, force_element_volume_geometry, and force_element_volume_time.