Class 13 material is a designation of the Resistance Welding Manufacturers Alliance (RWMA). It describes a pure tungsten material with good conductivity and excellent strength at very high temperatures and forces. Class 13 is the most heat tolerant of the refractory materials. It is used for cross wire welding and electrobrazing as the electrode material. High heat and force applications are where it does it best job.
Class 13 is extremely hard but its ductility is very low. It cannot be machined. It must be ground to required shapes. It is very heat tolerant and can glow red and deliver force and current to a part. Think high heat and force for this hight temp work horse.
The product description and properties of Class 13 can be found in the Resistance Welding Manual section 18-3, available at RWMA/AWS.
There are other Class materials with other properties shown in the above reference. Their properties allow for welding work pieces at lower heat and force requirements than tungsten is used for.
Reference: RWMA - Resistance Welding Manual 4th Edition
AWS J1.3 - Specification for Materials Used in Resistance Welding Electrodes and Tooling