Selects the area for which the condif_radiation record with the same index should be applied. This is only available for linear elements.
In 1D, the radiation heat flux is 0. In 2D this option is available for all elements. In 3D this option is only available for linear elements. the triangle between the 3 nodes will radiate heat. In stead of a number of nodes also, by example, -geometry_line 1 can be used in 2D, indicating that the nodes on line 1 radiate heat. See also: condif_radiation.