States which unknowns in which nodes get prescribed values by adjustment of the node_dof records. The item node_range represents a range of node numbers. In stead of a node range also, by example, -geometry_line 1 can be used, indicating that the nodes on line 1 get the prescribed boundary values. The items unknown_0 etc. are one of the primary unknowns listed at dof_label.
For a specific index, only one of bounda_force and bounda_unknown can be specified (thus either Neumann conditions or Dirichlet conditions).
Example for discrete node forces in y-direction on the nodes on a line:
bounda_unknown 0 -geometry_line 1 -vely
bounda_time 0 0. 0. 1. 1. 100. 1.
Normally you only should specify boundary conditions on principal unknowns (like velocity, temperature, etc.) and not on strain, stresses, etc.!
See also: bounda_time, bounda_sine, force_element_edge and force_element_volume.