With this option you can use a 2d beam in a 3d calculation. With first_direction and second_direction you can set the plane in which the beam will act; set these to either -x, -y or -z; set first the smaller one (so e.g. -x before -y).
Example of beam acting in x-z plane:
group_beam_inertia 0 ...
group_beam_plane 0 -x -z
Please realize that the beam mechanically still is a 2d beam (it does not model double bending or torsion); it just can act in a 2d plane in 3d space.
Any element output like element_beam_direction and element_beam_moment is defined in the plane of the beam.
If this record is not specified for a beam in 3d, then -x -y is assumed. In 2d, this record cannot be specified (the beam is always in the x-y plane).
The index specifies the element_group, see element_group.