LINK31 is a 2-node, nonlinear line element that calculates the heat flow caused by radiation between two points. The element requires you to specify, in the form of real constants:
An effective radiating surface area
View factor
Emissivity
The Stefan-Boltzmann constant.
Limit your use of the LINK31 analysis method to simple cases where you know, or can calculate easily by hand, the radiation view factors.